The short answer is don't tell her.
But remember that sooner or later, she is bound to find out. No matter how you hide, no matter what you hold back, sooner or later, she will find out.
Stealth only works for so long. It is a temporary measure at best.
But what would be the harm in telling her at an appropriate time? This way you are in control of the situation, and can set the stage. Otherwise you would be in a defensive position. What will hurt the relationship as much as the revelation is the fact that you felt that she could not be trusted with this part of you.
Think about it. You mentioned that she wants to be friends. Are you feeling that it might become more than that? If it does, then the results of her finding out instead of you telling is even more harmful.
It is a hard path that we walk, and we always worry about who to tell and when. That is the unfortunate truth.
-Sandy