First, what a vicious, ugly rumor for your friends to be spreading about someone. They have no idea if there is any truth to it, and given that it's probably not true (even in part), it's ugly to spread such things.
Even if she had every STD possible, nothing you did with her is any risk at all.
Why not have a talk with her about STDs and testing? You could just get tested together - lots of couples do - and find out what either of you may have that might affect the other. Her past is really not your business, but if she has anything currently, you should know that. She should also know if you have anything currently.