Basically a tortoise crept into my parents' country house garden last year and my mum has "adopted" it. She leaves appropriate food outside and makes sure there is some water nearby. The garden is 6-7,000 feet big.
Is the animal suffering any distress by being in captivity?
If released, would the animal still be able to survive in the wild?
No, he's/she's probably relieved of a lot of stress, just by being there. And your mom isn't preventing the tortoise from carrying on as usual, is she?
Maybe get a good vet to stop by and have a look at the situation.