I'm not really sure how this plays out or if it's even an actual heel turn, at least as of right now.
One scenario could be that Zayn interfered because despite all that Kevin Owens has done to him, he couldn't stand by to see the father of his godson possibly have his career ended. I could see something like this for Zayn where he was so torn because he knew that someone would get seriously hurt and he had to choose. As a result, maybe this leads to a heel turn whenever Shane demands an explanation and/or punishes Zayn for what happened.
A more obvious scenario is that Zayn "finally woke up" and realized that maybe Owens had been right all along. The last time Owens said anything about him, on SmackDown Live, was a reminder of their careers in WWE; despite Zayn being signed first to WWE, Owens is the one who's gone onto succeed because of his ruthless tactics. Owens took the NXT Championship at Sami Zayn's expense and after being in NXT for only a few months made it to the main roster. In that time, Owens has been Universal Champion, 3 time United States Champion and 2 time Intercontinental Champion; he's spent almost his entire time on the main roster chasing after or wrestling as a champion.
It'll be interesting to see where this is going, though I kind of hope that they don't really give us the full explanation or direction where this is headed in one night.