Yes, as cliched as it may sound but a lot of couples start to feel a bit tired after being married for 7 years and the spice kinda goes down a bit. And also your thinking is actually right. A lot of people conclude that their marriage becomes more exciting when they have an affair. In your case since it will be a polyamorous situation, it will work even better.
Bring your best friend into confidence first
Now what you need to do is to first find out if your best friend is up for it or not. If not, that is nothing to worry because in that case you can try to find someone else. Once you have your friend’s support, you can devise a plan together to seduce him. I like this approach because she is your best friend and hopefully you can talk honestly with her. Plus, by doing so, you are not expecting him to do the heavy lifting (a lot of men are simply terrified of going too far because they are scared of their wives; in any case you want to join them in the bedroom). You both will plan it out and do the work and then he just has to pretend that it was not him but you.
Alternatively, you can give him the green light to seduce your friend by saying that it is your gift to him or that you too also have your eyes on her for years and would like to join.