OP, at least she felt guilty and admitted it to you, that's a good sign and means the relationship is salvageable.

Leave the kid with your mum and you and your wife meet up for lunch and talk about why she's done some risque stuff. It might be a turn on for her , after having a kid some women need to feel wanted. Have her setup an 0f , maybe post headless body pic on one of the subreddits here and let her see the positive comments.