Kelley, I like Tamara's suggestion of fostering, that seems nice and probationary. Don't fault the person for seeking companionship out of loneliness, even depression, he/she seems to have hit on a solution that has the potential to be mutually beneficial to them and the animal.
It seems that he/she has given themselves a lot of self-punishment - there is nothing off-hand about the admission, there is remorse and it seems to be genuine, and after all, they didn't have to admit it at all. Suggesting volunteering is always a great one, but I feel as though a response that acknowledges the person's concern over the issue, and that it mirrors yours, would be appropriate. Ask him/her how things have changed if faced with the same situation now. Ask him/her how they would have handled that situation again if given the opportunity. Then make your determination of whether to proceed. Leslie

