Has he promised no contact? What steps has your MC recommended he take to maintain no contact?
This ^^^. Look up "infidelity transparency no contact" and do some research on transparency plans -- what they should entail, how to insist on one, etc.
Affairs are HIGHLY addictive (the altered brain chemicals even show up on CAT scans!), and you're never going to get anywhere until you separate the addict from the source of his addiction.