You're right. You should live your life with no regrets. But you should also live your life with knowing that one day you will have to answer to God for how you lived your life. (Judgement day will come whether you believe it or not. Sorry)
Just like you have to obey the law. If she feels guilty about breaking God law, then she shouldn't do it.
But I don't care what she does or what you do. I don't think telling her to throw her religion down the drain just so she can have sex with another girl is the right answer.