Can't you just change the chip address to yours?
We had a nightmare here with Merlin, the vast, vocal white Siamese who the caretaker has faithfully fed and sheltered for years. We've all piled in with food, cat house, vet, etc. When Merlin's owner was contacted via chip, the bloody woman wouldn't give him up, despite Battersea Dogs' Home pointing out everyone else was keeping him alive. Esp caretaker who is easily 300 years old and has to get Merlin up with a gong at 6am as they are both deaf. Anyway, there's nowt we can do except look after him.