There are a number of ways to answer this question, but the short answer is that I don't think it is going to be worth your while to sue her.
You did not have a written contract with her (I'm assuming) stating she owed rent or was to reimburse you for expenses paid on her behalf. So any claim of money owed under contract would be based on oral "contracts" you may have/may not have had. There is a theory of "unjust enrichment" in which you could say she was unjustly enriched and should be required to pay her portion, but she could counter with a number of affirmative defenses that I can think of. The bottom line is, I don't recommend suing her.
I welcome any ideas from my colleagues.
I agree with Mr Cooper, this is probably a long shot at best. Its going to be particularly difficult to get her to pay some non-agreed upon rental amount, with no written agreement. I'd chalk this up to a learning experience. Wish there were more options for you.
The question is not whether you agreed to let her live for free or whether you specifically agreed to pay for her trip. The question is, did she ever agree to pay rent or to reimburse you for the payments you made?
If you never had that discussion then she has no legal obligation to pay you anything. You should have had that discussion with her before you paid for the trip and before you let her live with you for over 2 years. The current circumstances make it look like the only reason you're bringing it up is because she is not dating your son any longer.
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