I can only relate my personal experience here (and knowing that my experience is pretty common):
If your spouse is having an EA or PA and reaching out to you for "friendship", then their needs are not being met by the affair.
If you take friendship off the table, then they will have to asses whether or not the affair will ever meet their needs. I and others took friendship off of the table because... it just wasn't going to really happen in all honesty. I was not going to have an emotional affair with my own wife/ex-wife while she pursued her emotional affair with somebody else. If the marriage was over, then no woman in her right mind (good boundaries) would want to be in a relationship with a man who was "freinds" with his ex wife.
I would caution people to examine the reality of their own situation. Being there, being friends, etc can actually prolong an affair, do more damage to the marriage, and result in more divorces.
I wasn't nasty. I was honest. Friendship wasn't going to happen. That was a boundary to protect me. If we were going to divorce, then I needed to be able to move on and possibly meet somebody else who appreciated me more, and I couldn't do that with an ex-wife buddy in-tow because women with healthy self-esteem and good boundaries won't tollerate that.
Also, I was not going to date my wife while she dated others. Again, that is another personal boundary: I don't date women who are unavailable.
The biggest problem in my marriage was caused by weak boundaries: my wife's EA.
If the experience taught me antyhing it is that you cannot take healthy boundaries (both within the marriage to protect the partners and external boundaries to protect the marriage from those outside of the marriage) for granted.
M-47,W-40,No kids D-filed 5/27/2010 Piecing - 10/21/2010 -=Soon to be banned=-