I totally understand why you are upset, she asked a huge favour from you based on a promise she KNEW that she couldn't fulfil, but you didn't
Asking to borrow that sort if money is IMO cheeky too, lay a conversation that allows you to offer by all means, but to put you on the spot by asking out right is too much, especially when she would know that you aren't rich enough to swallow such a huge loss & in the circumstances of her knowing she would be very unlikely to be able to pay, it would effectively end your friendship & put you under a lot of stress anyway
So no, contract or not YADNBU
BUT - no real harm has been done because you did say know, so no need to overreact to something that hasn't actually happened because you had the good sense to foresee there might be problems anyway & said no.
I would have a chat with her about it though, let her know your were upset that she asked you knowing that her circumstances were about to change & that had you felt able to say yes to the loan, then it would have created an intolerable situation that would have put immense pressure on your friendship & that she knew that when she asked you & that you feel very hurt that she would do so.
It does sound as if she's floundering big time & not given a second thought to you out of desperation - are there possible reasons for her change in ability to manage her finances well- maybe that's something you can discuss & suggest ways for her to get financial advice too
I don't get why the issue of a repayment contract makes a difference, had you said yes to the loan then I'm sure you would have been sensible enough to do just that - it wouldn't have made a jot of difference to her ability to pay you back though, so without a messy court case to reclaim your dept, it wouldn't have been worth the paper it was written on
& I'm speaking as someone who DID once loaned a friend a similar amount of money, with contract, though I knew he would never stitch me up & was in a situation to repay, I also knew that if he did there was little I could do, nothing that would prefect our friendship that's for sure.
Talk to her, sounds like she needs it - gawd knows why, but I find myself wondering if there are drugs involved 