Isn't BFF Best Female Friend?
If the drinking continues you'll quite likely need to deal with more of this. Whatever he's promised or how guilty he feels will not get in the way whenever he's loaded and gets a chance. Drunk moral standards are legendary for their flexibility.
If he stays sober and lives by principles that require it, he could be faithful to you the rest of his life. Pretty much hinges on what he does to make that possible. You may want to encourage his heavy participation in AA from the viewpoint of self-interest.