This is a good idea to also test the waters. But they still should discuss strict boundaries before going to a swingers party.

This is a terrible idea. You would be running the risk that he regrets it and then can’t look at you the same. Fantasy does not always translate well to reality.

But if you are going to do it, for the sake of your marriage, you might want to go to a sex therapist together first so you can discuss boundaries and all the post-bj feelings that could come up.