Do you think my friend is stupid for doing this?

Yes, it is rather stupid for her to be doing that. No one should start dating someone with the intentions of dumping them... That's freaking ridiculous. I'm sure your friend doesn't mean to be heartless or make stupid decisions, so maybe you could suggest that she just tell Michael that since she isn't going to be around this Summer, they shouldn't go out at all. It's really good that you're looking out for your friend and that you care enough to not want her to make a bad decision :)