Was the diagnosis made by a doctor? If so, then I would expect the doctor had blood work done to check for possible causes, including HIV. Bell's can appear without HIV being present, as well, and in fact was a recognized condition prior to the spread of HIV.
There are some limited treatments that are most effective during the first 10 days following initial onset of symptoms, so if he hasn't yet seen a doctor, encourage him to do so immediately. I see no reason for you to raise the HIV issue, which the doctor should do.
Bell's symptoms sometime spontaneously disappear without any treatment, which I hope your friend experiences. But he should not count on that, and again I suggest you urge him to seek an immediate medical evaluation, if he hasn't already done so.