Something makes me think that different generations have different attitudes. Remember, your MIL has forgiven him once for an affair - and hasn't been eaten up with bitterness, subsequently. The shame of having people know your business, and someone younger knowing that their own marriage is better and truer than your own, might be more devastating than understanding that your husband is......what?
It's not like you overheard him saying he would leave his wife
Or its the case that he has missed a family anniversary celebration to be with his bit on the side
You don't know how involved he is with the woman.
Your PIL might not be what they seem. She might put up with what she sees as a dalliance; or maybe, being crude, your FIL can't get it up, so she knows there won't be actual sex.
MIL might have a good, breezy, cheery front, but be shrewd and harder underneath, and want the life she has with her husband, and the respect of having been married and pillars of the community for years, with or without playing away which she thinks isn't important to him.
You could tell your husband, but be aware of what you don't know as well as what you do. Let him form his own opinion without ramming down his throat that his dad must be cheating and, in the process, making pillock's of his mum and everyone really, because you are all less easy in the heart than you were before.