Do i tell my gf what i done with her friend?

I am using my gf yahoo and hope she don't read this.Anyway My gf 18 year friend from out of town was staying for a few days.She had her own bedroom.I am 25.About 3 am i went to get a snack.The friend was in the kitchen.We was both wearing a robe.She open her robe revealing her naked body.I was staring.Any guy would as she is hot.She going be in penthouse magazine soon nude.She started to ************.Then she said she want to loose her virginity to a older man.We went into her bedroom and had sex.I don't know why i did.I never cheated on my gf and we been together for 5 years and going get married soon. my gf never woke up.The next night i was in bed with my gf who was asleep.We both sleep in the nude.The friend came in and ************.I told her to stop.She wanted me to go in her bed and i said no.my gf a sound sleeper.Do I tell my gf what happen or keep it a secret?