Need help with pit mix

Sister got a 4 year old dog from BARC, took it home and he didn't do well with her old boxer. Less than 2 days later BARC refuses to take him back. She cannot keep the dogs in the house together moving forward. What other options can I help her find? He's sweet and beautiful, unless her boxer is around. She's a total mess. She picked him up after her other boxer was killed while on a family vacation. This is too much, and I need to get him moved for her.