I completely expect Zach to leave Heather if she lives. The only reason Zach has stayed is because Heather provides him with shelter, food and dildo money. That gravy train just ended. It won’t be immediate because Zach doesn’t have anything to fall back on, he suffer for a few months to a year as they’re forced into a smaller apartment and he has to rehab Heather. Maybe he’ll even be forced to get a license and a job. But as soon as he can say he tried and he has somewhere to stay (at that point, he’ll be ok with moving in with his own Phil to escape), he’ll be out the door.
Zach does not love Heather. He barely tolerates her. This is a strain on a healthy relationship, let alone the unholy parasitic relationship they have.