She will text him playing the victim.
It sounds like he may be a "rescuer" and his ex has him dangling on a string, constantly needing to be rescued. He may well not be having an EA with her. What he has is worse, it's a co-dependent friendship that sucks up all his attention because it satisfies his insecurities.
You cannot compete with this friendship because you're too self-sufficient. Even with the health problems you just don't need rescuing often enough, so you can't scratch that constant itch he has for saving the day. If he cut off this friendship (which isn't going to happen), he'll just go looking for another woman to rescue.
Nothing you can do or say is going to change him - he needs to work on himself, probably with professional help, so he can learn how to have healthy relationships. If you were feeling kind, which you understandably may well not be, you could explain to him that close friendships are normal, but that this is not a normal close friendship. She's using him, and as long as he lets himself be used then he won't be able to have a proper romantic relationship.
But I expect he won't what to hear that, because too much of his self-esteem is wrapped up in the friendship.