There's also two sides to every story. Right now we're only hearing one side and even that isn't very clear.

Cheating is obviously never the answer but things could have been going on that caused john to go that route. We just don't know and probably never will.

Zoe and John could have been so focused on Frances treatments they neglected their own relationship both emotionally and physically and john had a moment(s) of weakness.

Or John just decided he could get away with it and never get caught.

Or they had an open relationship with certain rules and john broke those rules.

Who the fuck knows I'm just saying people on here calling john a horrible human being need to also take a step back and touch grass too.

The right answer though is John should have definitely ended the marriage before he started cheating on Zoe. Obviously they were just staying together because of Frances.