The following video is a great example of people not understanding dog communication. She thinks the dog is "Purring"! What do you think?
The vocalization is growling. Growling means either go away from me, I am uncomfortable with this or stop doing that. This combined with the body language and facial expression of the dog shows you it is growling and is NOT a happy sound. He means it. The sad part is that when the dog finally bites her she will think he "snapped" with no warning and for no reason. This poor dog is TRYING his best to communicate here!
Yes some dogs can have happy vocalization sounds in play. Unfortunately this is not one of them.
The other scary part is that she says she is a foster home. I would be very fearful of her qualifications is she can't even recognise growling. Especially in such a large and powerful breed. Do you want her chose which home this dog should go to? I bet she would place him with children who love to hug dogs. Many dogs dislike hugging from strangers. It is obvious that this one is no exception.
I wonder if anyone temperament tested this dog at all before putting him in this foster home. I know it's not always the perfect test of a dog but it can be a start for some. This behavior might have come out in a SAFER test.
I feel sorry for this dog. He could end up dead all because someone didn't listen to him. Hopefully he doesn't hurt someone to badly before they figure it out.