Good for them! I have no problems with this and can totally see it working.  

I had a similar relationship with a guy in Alaska one summer. He was gorgeous!!! Excellent in bed. I met him and we became lovers for a couple months. (I am polyamorous myself, so there was no commitment- we both still dated and slept with others during that time- although we were together most of the time, so very little time for others.) Then the last month I was there, one of my best friends (who is bi and was a lover from time to time) came to stay with me. I introduced them and we all had fun together! It was a blast, and one of the best summers of my life. And probably theirs, too! Yum, yum, yum!!!