Biting in declawed cats can be for any number of issues, but all go back to pain. A cat will still play and be sociable, but when the pain spikes, he will bite. It could be paw pain from: bone chips, nail regrowth, festering abscess in the bone, hammer toes, contraction of the toes, bare bone sticking out, residual nerve pain.
Pain in the legs, both front and back, hips, spine, shoulders, from the altered abnormal way the declawed cat now has to walk and stand. The abnormal gait happens even if any of those paws issues aren't present, because a cat walks on his toes, and when the toes are cut off at the first digit, the cat is crippled for life.
but any of those paw issues can make the body pain even worse.
Sounds like a good thing he has become her foster fail, because he needs someone who can understand what is causing his issues, and help fix them.
She needs to take him to the vet for x rays, not only of the paws but the rest of his body too. Preferably to a vet who does not declaw, if at ALL possible. Declaw vets are feeling "persecuted" these days because the movement against this evil is growing..
I recommend she look for the Paw Project Director practice in her state, if there is one, and try to get an appointment there first.
http://www.pawproject.org/no-declaw-vets/If he needs paw repair surgery, he may need to travel if her state doesn't have a vet who does these surgeries.
If it's "only" arthritis..there are supplements he can take, and diet improvements she can make,.
In either case there are probably environmental changes she can make to help him feel more comfortable.