S.G.
I don't think you need to be careful about talking about this with your husband. Just tell him what's been going on. Like you've said this guy has not done anything wrong he just goes a little far. I think that's easy for anyone (MIL, friends, siblings) to do if they are around you a lot. This guy probably thinks he's helping you out and probably feels like he knows your parenting style since he's there all the time. Just tell your hubby what you told us. Tell him you really like the guy and respect him but don't want him to have to take on the responsibility of parenting. Right now this is an easy situation to fix, just don't let it go on any more.
Don't be concerned about hurting your husbands and his friends relationship. I really don't think this guy will take any offense if your husband talks to him.