I'd lay out everything with him. What degree he's going for, if the school has that degree and/or is actually good with that subject (there may be a difference), what the costs are, how HE is going to fill the gap, what will he do if they break up, etc. He needs to find out how the transfer works, what (if any) of his credits will be accepted, etc. And what if he doesn't get accepted to that school? That's not a guarantee, either.
My stepson went to a different school than his GF and they broke up. They chose schools based on their futures, figuring as you put it that if it's meant to be, it would work out.
If he wants to change, and people do transfer, then he needs to look at the whole picture. Will this put him back a year with credits that won't transfer? Will this mean he gets a job or a loan on his own dime to cover the new costs? My husband gave my stepson a budget. This is what we can give you. The rest is up to you, be it via your mom, work or loans. I would not just pony up money because he wants to follow his girlfriend. If he wants to make adult choices, he has to take some adult responsibility, in my opinion. Parental payment for college is a privilege, not a right.
If my stepson thought it out, we'd probably give him our blessing. But he knows our $ support runs out after 4 years so if a transfer or changing majors puts him back a year or a semester, that's something he needs to consider.
Oh, and I agree with the first year mark. We asked my stepson to give his major one year (because his program is hard to get in but easy to get out of) and if after that he was just sick of it, we wouldn't hassle him about changing. But we didn't want him to quit because of one tough semester and we also didn't want to make him stay til he'd be stuck with credits he couldn't really use later. He's doing an "exploratory" semester right now where he's trying out a bunch of different subjects and will re-declare in the spring.
It's also a good point to ask if she's considered changing for him. Or is it a matter of him being overzealous and changing for her when maybe she doesn't feel the same way he does?