T.S.
Hi Shelly,
I don't know your situation or the circumstances of your divorce, but I can tell you what works for us. My ex and I separated when our son was 3. While we decided that we were not happy being married to each other and just plain old did not agree on many things, one of the things we do agree on is how much we both love our son. We made a promise...actually it was a written agreement that a counselor helped us to write about our commitment to treating each other fairly and with compassion in the best interest of our son.
We have 50-50 custody. I have him on Mon and Thursday and every other weekend. We live about 15 mins apart so this works pretty well for us. Our son has adjusted pretty well too.
As far as letting him have overnights with his dad. I guess it depends on the whole situation with his "friend." My ex and I agreed that we did not want a string of boyfriends or girlfriends to our child as it can be confusing for them. If we were in a long term relationship we would both agree it was time to introduce that person or the relationship to our son. In my ex's case our son already knew his fathers girlfriend from church, they just needed to explain that daddy and his girlfriend were now a couple and what that means. So back to your husband, I would think it was ok as long as he was not sleeping in the same room as the friend. Of course I don't know all the details. As long as your decision has nothing to do with payback or punishment of your husband, I think you will make the right decision.
My ex and I promised that we would never do anything to interfere with our sons relationship with either of us. This means we will not move more than 1 hour away from each other. That we respect each other as parents and each others rights. We do not talk bad about each other and never fight or "discuss" each other in front of our son. We try when ever possible to provide a united front.
I hope this helps and best of luck to you and your son.