Wow, what a moral delimma.
On one hand, notifying a guys girlfriend that he has HIV? Morally, do it. Legally.....?
See HIV carriers are protected legally from discrimination. Since we have no cure, we have focused on maintaining that person who would have otherwise (without advance treatment) died. Therefore, by prolonging thier life and not curtailing the proclivity which created thier condition is of questionable benefit to the general public.
It is a sad state of affairs.
As an example, look at Magic Johnson (?). He had sex and infected hundreds of women. Yet today he is a 'hero' of the movement.
The edict "The needs of the many outweigh the needs of the few" doesn't wash well in America.
SO, in your position, I would let the person know, ANONYMOUSLY if posible.
(This is a hard issue to broach, as many of us have friends and family that are HIV positive.)