I'd be very disappointed if my daughter did that to me. Not sure what I would do, except to say that it is "unacceptable" to do that to family. She was consulted in advance. She should NOT have accepted another invitation because she already had plans. Responsible adults DON'T do that. It's just not nice. I raised my kids to be NICE and to follow through on obligations, whether they want to or not.
Now, of course, you can't "make" her go. The snide momma in me would probably say "hey, I know I said I would pay your tuition/phone bill/apartment rent (whatever), but I changed my mind...I got a better offer for my money. So sorry about that."
We had a long discussion over spring break about NEXT year's spring break. Both my college kids have the SAME break (they did not this year), and we said "we want to go to San Francisco....are you in?" Answer from both was an excited "YES!" Once I book the airfare (which I have not done yet), it's a DONE deal, and they will be going or suffer my wrath, which might well include saying "too bad about that tuition bill." Is that fair? Probably not. But, neither is it "fair" to break commitments. Actions have consequences.
ETA: Would I for real not pay tuition? Probably not. I think my real life consequence would be to force the offending child to take the federally guaranteed student loan for remaining years. So far, their Dad and I have footed the entire bill, not wanting to saddle them with debt. If they respect us so little that they will make, and then break, commitments to the family, I would similarly say they are no longer entitled to the GIFT of a free college education.