If the guy made a good-faith attempt at buying a ring he thought his fiancee would like--I'm not sure if we can really say; spending only $15 and refusing her suggestion that they shop together makes me wonder--then honestly I would probably keep it. But, I'm not a big jewelry-wearing person. If it was too big or awkward to wear on my hand, I could put it on a necklace chain, or only wear it for special occasions when getting "dressed up."
Most of the time I see women wanting to be "surprised with the perfect thing" and mad when the guy can't read their mind.
It's totally normal to go to a jewelry store and pick out several things, then one person leaves while the other makes the final decision--the stores do that all the time. Or, if one person can send the other pictures of what they want, the buyer could show them to a salesperson and get advice (probably better at a dedicated jewelry store). Or even shopping and picking it out together, or both parties being involved in designing something custom. If the style really matters to you, because you often wear jewelry and you have a style, then frickin' give serious input. (Like I would never let someone else choose my wedding dress, because I have a certain style of clothing I'm comfortable with, and odds are they would not pick out something I'd be comfortable wearing even for a few hours.)
Or else, take the gift you are offered with good grace, even if it's not "perfect." Again, assuming a good faith attempt. The entire point of an engagement ring is to be symbolic--your future spouse isn't going to be perfect, either, but you take them anyway.