well interesting enough, never had this happen, and know many SP's with girl friends who do not know that they are in the SP business. Far more common. In fact many many SP's out there who do not meet other SP's. So if your girlfriend's friend is an SP and you like her, don't worry about it. Chances she will be become your new girlfriend are pretty slim, so don't count on other developments either.
Just part of the business, go for it if you wish. and don't worry about reprecussions. She isn't going to tell, she will want more business from you, and girlfriend relationships often don't preclude this type of thing, SP or not.