I'm sorry you are dealing with this.
He is a very beautiful dog. A lot of dogs are leash reactive and aren't truly aggressive, but when it's coming from a big dog, it often looks worse of course. Not saying it's right -- but I've read alot about people on here and Yorkies who bark like crazy at the window and lunge on the leash at other dogs, but they are not truly dog aggressive, there is a difference.
Have you ever seen this dog in a setting off leash with people or other dogs? It may very well be friendly with dogs and people. I dogsit a Rottweiler who has a leash reactive problem sometimes, he gets overexcited and barks when he sees another dog approaching on leash, but he's very dog friendly and goes to the dog park every week and gets along with Jackson. But he can look very intimidating to a person of course. It's something that just needs to be worked on with some training (which I help the owners with, and he's improved a lot).
I hope the situation gets worked out. It sounds like she doesn't care enough about the dog to get it the proper training and is taking the easy way out by rehoming, which is sad.
He'd probably be a fantastic dog in the right hands.