My own thoughts:
1) No mention is made of the writer's partner, if one exists, or other adults in the kids' lives. When I was a kid, I loved to play pretend, but mainly did it with my dad, because it was pretty obvious my mom wasn't into it. But, my mom was much better at other things. I would have told the writer that if there's another adult in the kids' lives who is good at playing pretend, shove that duty off on them, and concentrate on other things. Yeah, there is always boring and unpleasant stuff you just have to do as a parent, but you don't literally have to play pretend with your kids, as long as you give them the opportunity to do it with someone. Based on my own memories, I'm betting the 6-year-old has already started to pick up on his mom's dislike of the activity and feels conflicted about that, and will soon stop asking her to play along. I would suggest the mom look for other imaginative activities she can do with her kids (which she might already do), like reading outlandish books (some people like doing the funny voices), making freeform crafts or Lego projects (rather than just following directions), or just talking about silly things like, "What would you do with a pet dragon?" Something that ticks the "creative, imaginative" box but without completely boring her to tears.
2) I felt the columnist's advice was pretty soft for a teen who is literally flunking most of their classes--that's something I would find pretty alarming and would want to get to the bottom of. Of course, I can see not making a huge deal of it, because kids can be stubborn and you might poison their minds against school entirely. I would suggest the parent talk to the school guidance counselor and the student's teachers for specific advice. The ADHD (etc.) testing is good, or perhaps gifted testing if the school has resources for that. See what resources the school has generally--can the kid have more rigorous classes, more individually-tailored classes, smaller classes, more hands-on classes, etc.?
The student genuinely doesn't seem to have much homework to do--I know some teachers can go overboard with busy work, but in this student's case, having more tasks to complete might help her stay focused and reinforce the material. I know when I went to college, it was a big shock to have basically no required homework, with the only grades coming from major exams and maybe a couple of quizzes or papers--I quickly learned I needed to do the "recommended" homework anyway, so that I was prepared for the exams. But it would be pretty tempting to just ignore all that non-required homework... until you got to the exam and had no idea what it was talking about. A college student should be mature enough to make the right decision, but a 15-year-old, not so much. At least, this would be something I would be asking her teachers about.
I'm also concerned about the "toxic friends" comment. Forbidding a friendship usually has the opposite effect of the one you want; but I would see what the school could do about separating my student from those other kids, from classroom seats to a different class schedule. Also, are there any extracurriculars the kid is interested in, either at the school or in the community? The writer says she's interested in a photography class, which they're holding out as a reward for better grades, and the columnist suggests they let her take that class anyway, since it's something educational she's interested in. Just thinking longer-term, even if the student is interested in an activity that's not strictly educational--sports, maybe--I would give her the chance to participate in the hopes that she might make new and healthier friends whose peer pressure/example would encourage her to apply herself more at school. Also many school-based extracurriculars have minimum grade requirements, which might be the kind of motivation she needs.
Since this is her first year of high school, I would also try to figure out what's different from her previous school, where presumably her grades were better. Maybe the teachers here do things a lot differently, or expect a lot more of the students than the more permissive previous school--a lot of kids suffer during their first semester in college, for example, because they're expected to be much more personally responsible and show much more initiative than they ever did in high school, and that can lead to them not studying enough. The transition from junior high/middle school to high school could be similar in this case.
3) The mentally ill, racist grandma is a tough one. The family has already made the decision to visit her, when some people would just never have gone back; but, they are staying in a hotel and not with her, so that's a great way to keep some distance and independence. I would say, when the family goes back to the hotel at the end of each day, definitely open a conversation with the kid about anything weird or bad that happened. I remember being that age and having to visit eccentric relatives, and anything weird was just kind of swept under the rug or explained away dismissively--I was definitely aware that I should not bring up the topic myself, even if I was confused. And I think that did everyone a disservice, because in some instances (looking back) the differences were due to people who just had a different perspective than I did (coming from a very poor background where you didn't waste anything, for example), and it would have built empathy in me to have that pointed out. And other times they were just being weird or racist or something, and it would have been good to have that acknowledged, even in private, because as a young teen you can end up over-thinking and wondering if the strangeness is you rather than other people. (I did once win a "weird relative contest" among my friends in high school.)
Also, the letter doesn't go into this much as it focuses on the kid, but as a wife I would want to know a lot more about how my husband has been keeping up with his mother and how, exactly, her behavior has been. And the writer may know that already, and have only agreed to this trip because all evidence shows the grandma has indeed mellowed. But I would want a clear understanding with my husband that at any point, I could cut the trip short if I felt grandma was too much--whether that's everyone going straight home, or just me and the kid going out to see the sights while the husband visits his mother solo, or what. They really don't know what she's going to be like, and just in case she's worse than expected, or has a bad day, I want it understood I will take my kid and get out of there rather than expose him to repeated horrible or scary comments.