I think a friend can point out patterns. Like, "It seems like you complain pretty often about him being irresponsible with money. There was the time with the electric bill, and the time with the video game console, and the thing with his car, and... But now you're saying you want to move in with him and make a financial commitment?" It's not so much "warning away" as saying, "Hey, you might have missed this pattern. How are you reconciling it?" And you might find that your friend has just been exaggerating their complaints and actually thinks their SO is pretty responsible; or maybe they are cool with joining up and being the person in charge of all the money, so it's fine, at least in their minds.
But, I find that a lot of times people are startled when you point out a pattern. Patterns are really hard to notice when you're part of them. Sometimes it really gives them something to think about, like, "Hmm, yeah, he does do that a lot." Other times, they realize the slanted perception they have been giving YOU, when it doesn't really reflect how they actually feel. Like they always tell you the complaints but never the good things, so you end up thinking their SO is worse than they really are. Like I used to be friends with someone who, after she had her first child, would tell me how sweet her husband was with the baby. And I was like, "That's so awesome, you complained about him so much during the pregnancy I was worried he would check out and not be very hands-on." And she kind of gave me this cold look and said, "I wouldn't have married him if I didn't think he would be a good father!" Well, excuse me, you spent your entire pregnancy complaining about how he was anxious and ignorant and lacked common sense and couldn't come to grips with it. Why would you think I would conclude anything other than that about him?
Nowadays to me it's not the marrying that's a big deal, it's the having kids together. Once you have a child with someone, you are stuck with them. Even if you get a divorce, unless either you or they completely check out of the child's life (sometimes a good thing!), you are going to be dealing with them for the rest of your life. Plus a child can easily be an innocent victim of the adults around them, whether it's parents or grandparents or whoever doing a power play. There was this one guy who I worked with for several years, and I thought he was extremely immature and insecure, and even misogynist and racist but almost more just as part of his immaturity and insecurity--like, knee-jerk, must make fun of someone else to avoid getting made fun of myself. And, he was way too old to be that immature--he was around 30. He left our company and moved someplace else, met this woman, moved around with her, they ended up getting married, and about a year ago I heard that they had a baby. Until that point I was just kind of rolling my eyes--he had become a joke in our office, like ha-ha, remember when he said this or did that, what an idiot, ha-ha. But after the baby I was like, "Okay, this is serious now, and could be very sad." Granted I haven't seen him for going on 10 years, but it's hard for me to imagine what kind of miracle worker this lady must be to have transformed him into someone who could be a healthy parent. I'm sure I'll never know, because I won't ever seen him again beyond Facebook probably, but sometimes you just see stuff and are like, "Who thought that was a good idea?"