"the friend is younger and very immature so she has no interest in hom outside of the fact he is hot-"
So what if he becomes more mature in a year or two? Then he`ll be perfect for her. This should`nt be the reason it`s OK.
No matter who he is or what he`s like, she should really unconditionally love you and respect your marriage WHILE having sex with someone else ...IF that`s what you guys choose. What I`m saying is you should`nt feel "relieved" because he`s immature. The next guy could be more mature than you.
Of course they`ll be a next guy.