No need to take him back just because he's depressed. I think he would be a bad bet at this point. (I say this having forgiven my ex's infidelity right before our marriage, and a later infidelity 15 years later, only to have hm leave when he was approaching 50. They seldom change!)
That being said, if you want to give him motivation to pull out of his nosedive, think about what it would take to reconcile? I'd say - a year of living apart, therapy for him, mental health treatment for him, zero contact with other women, THEN maybe you would consider dating him. But divorce and financial settlement in the meantime (you could always get married again if the need arose).
Don't worry - he's not likely to do all that. But having clear boundaries about what it would take for you to even consider it is good.
Odds are good he's just having a fight with the OW, or is between OWs, and won't even pursue it with you.
This affair wasn't a temporary lapse due to depression, btw. It was a major character defect.
