Chances are ... Her parents already know that she is sexually active. If they didn't, then she wouldn't have to lie about it in the first place because they would not be asking.
A big percentage of men and women your age has already had sex, so I think she should go ahead and let her mom know what's going on. That way she could get on the birth control pill or something of that nature (if she hasn't already done so), so you guys can protect yourselves.
If you & your gf are honest with her mother, then she's going to respect both of you more in the future - even if she doesn't like knowing that ya'll have been intimate. Good luck! :-)
