I'm writing as somebody who has been on the therapist's side of the equation:
The risk to your son is minimal. I have a student who has bitten at least 5 or 6 times to the point he drew blood from me. I still have scars from incidents that happened months ago. This same student has bitten my para multiple times as well to the point of drawing blood. He has also bitten numerous times without drawing blood. At no point did he ever get sick from us. When I was first bitten, I had to undergo testing for HIV, Hepatitis, etc. It was a pain. There was never any concern from the doctors on his part. We would have loved to have known if he had those diseases for testing purposes, but because of HIPAA we could not ask.
The risk to he biter is minimal. At no point has my student ever gotten sick from biting us and this is not a one time occurrence. The mouth has a lot of defenses, that is why you can't get HIV from kissing. Short of him having a cut in his mouth and her blood mixing with his, there is little to no risk .(And even then the risk would be minimal.) The risk is to the victim as not only do they have to worry about potential blood borne pathogens, but also the myriad of bacteria alive in the mouth. Bites easily get infected and many times the victims have to take antibiotics.
Please, do not call her and ask her for that information. I'm pretty sure if the parents of my student had, I would not have given it to them. I would also have been infuriated at their complete lack remorse for their son's actions. Would you share your son's medical history if she asked? Probably not. So don't expect her to share hers.