I would definitely let her know that you don't intend to take care of them for free and that when she decides that she wants them back, she will get a bill for the amount you put into them. Let her know what you're buying for them before hand and the reason why. Let her know you won't care for them if she doesn't want to give them what they need to be taken care of properly. I don't mean go out and buy fancy stuff like you might do for your own animals but just things that they may not have that would be necessary for proper care. As a friend I know it's hard to straight out tell them "this animal looks thin/sick (insert any other issue here) and this is what you should be doing." Even if you did many people don't take kindly to it or ignore your advice anyway thinking they know better. I think it would be best to not offer any money for them or push the issue. Just be firm in letting her know that taking care of an animal the way it's supposed to be taken care of isn't cheap and since they aren't YOUR animals then she will need to pay you back or you keep the snakes. She may change her mind if she knows there are no freebies.