I agree with Kirst, I'd try keeping him in, at least till he's been chipped, and then make sure this women knows he's chipped, she might not be so keen to keep hold of him if she knows it can be proved he's yours. Vets normally have scanners and some rescues do too. I'd also ask her to stop feeding him - maybe make something up, like he's on a special veterinary prescription diet and anything other than that diet would make him ill. Just a little white lie!