Do i tell my gf what happen?

My gf friend from out of town was staying with us for a few days.One morning about 3 am i went to get a snack.The friend was there.We was both wearing robes.She open her robe showing me her naked body.I was looking any guy would she a hot 18 year girl who going be in penthouse mag soon.She open my robe and suck my penis.I am 25.She said she want to lose her virginity to a older man.i don't know why i did but i went in her bedroom and we had sex. me and mx gf room was down the hall and my gf never woke up. do i tell her?That was the only time i cheated on her and won't again