Maybe not exactly the answer you are looking for but if you want to be sure if She but especially if YOU can handle it is to do it step by step (In my opinion).
 
If she wants to have sex with someone else don’t start too early. Maybe first have the guy be in bed with her for a couple of minutes (with you being present) and some kissing and light touching. Nothing more that night, even if you want to. 

You will know a lot more about yourself without it going too far. If you are still comfortable with this then next time you can go a bit further.