How can me and my girlfreind safely put her out for her sex fantasy?

lol idk if its impossible because i woke up to my old fling on top of me once apparently she loved it lol i kind of woke up half way through she said it lasted about 10-15 mins but i finished before i was fully awake, i wish i had been awake but lol so it is possible!!!! If she really wants this to happen get her pretty drunk and fall asleep just make sure your not only way i could think to do it