I believe in being a good neighbor to good neighbors. My non horse owning neighbors once chased down, caught and returned my horses. But we make every effort to be good neighbors to them, they just viewed it as returning the favor.
In other situations, I have offered money, my services, brought baked goods or wine, and written thank you notes, in any case, being profusely grateful that other folks were willing to look after my animals in my absence.
The other situation is that I came home one day to find two strange horses in my spare paddock. Another neighbor had found them running loose and chased and penned them in our spare paddock and called the sheriff's department. Some hours later, the horses disappeared. No note, no knock on the door, no thank you, no apology, no nothing. Sorry, I just think that's rude, and a violation of good country manners. I would have felt obligated to introduce myself, apologize and offer to pay for any damages to lawns, reassure the property owner that the horses were vaccinated, etc. I will not be as likely to respond quickly or well if I see the same horses loose the next time.
In the OP's situation, it's tough. A lot depends on what condition the horses are in. If she charges board, she might just end up with the horses permanently. Someone who can't afford to keep their fences in good repair probably isn't a good candidate to pay expenses. I guess I'd get the sheriff's department or animal control involved and see what they have to say.