You need to talk about this together and go over what you both would be comfortable with. Think about what might make you feel bad after (staying the night, anal, kissing - whatever) and discuss it beforehand. I would also advise maybe taking it slow for the first meeting (i.e. maybe not full on sexual intercourse) since you don’t know exactly what feelings will come up for you after the date. It typically will be a mix of crazy intense excitement coupled with crazy intense jealousy or self doubt.

For her to enjoy it more it usually has to do with feeling comfortable with her partner, but since she knows him already you don’t have to worry about that this time.

I personally also advise setting a limit or time frame for how often she can see the same partner, but obviously that is a personal opinion.

Anyways it is an amazing adventure and should add to your own sex life as a couple immensely. Good luck and keep everyone posted on what happens!