Take her with you to meet the new guy.
Our first sheltie would accept NO OTHER dogs UNLESS they were shelties or sheltie mixes. If you wanted to know if a mix had sheltie in it, all you had to do was take Scotty near it.
We had fosters, got more shelties, got a sheltie mix and all was ok. Then hubby called and said there was a little beagle running around his biz and he was sure it had been dropped off.
I picked it up and while trying to find a rescue to take him, Scotty went into the bedroom and did not come out for 5 days. He was having NO part of that NON sheltie.
I have a friend whose dog didn't like other dogs but she wanted another one. I suggested she try him with another beagle. He was fine. And she was able to get a second dog.
Know it sounds weird, but it's worth a try.!