I'm not in the UK so anything I learn is from research. If it's anything like the US (and I have to believe that some common sense prevails ... sometimes) there is inequity in her working 2 days a week and him working 15 hours a day plus freelancing. How long have they been married? How old is the child?
You say she was having an affair while they were married. He is your partner now. Are they still married or legally separated? If not you both had affairs while you were/are married. But, no, moral conduct has nothing to do with child and spousal support.
Matrimonial actions, unfortunately, are very expensive in most cases, particularly when the parties are not in agreement. The "... toward the child's wellbeing" is VERY well said.
I think he has to look at the money he will need to pay an Attorney is money which he does not have to pay his wife. Do you know what I mean?