She's a 18 and an adult. She can make her own decisions.
Well an older guy screwing a young girls really not the the thing to do, being it was the daughter of a good friend that trusted you with her. But look at it like this, she's 18, not a virgin, had her own condoms, she wanted the dick, soppose you'd turned her down and later on she went out and got with a bunch of dopies, got raped, beat up or worse, how would you feel around her folks then? In my book you did nothing wrong.
As someone else posted, don't worry about it, you may well have saved her life, and if you used her condoms and didn't knock her up she has no reason to tell. You know now that you can fuck her, but if I was you I wouldn't ever ask her, sounds like she has a young stud or two to take care of her already.
O get over it. She's a big girl and so you had sex with her; yeah the parents would freek but it sounds like you would tell them before she ever would.
Well hell, it wasn't the absolutely smartest thing to do, but what straight guy could resist that?
She may well tell them in a heated moment in an argument, but there's nothing you can do about it now.
Savor the memory and go on with your life.