By D&D logic Wyll is not evil. Just because he is a fiend Patron does not make the warlock evil. You can still have a warlock that leans more over to good.
Also, the warlock could be rebelling or has already rebelled from their patron.
Unlike clerics, their power does not get just "taken away" in most cases, i've seen. All in all, he would most likely be in my party for a good playthrough.
It looks like they've walked away from the loss of power when displeasing god/patron in general in 5e. Individual GMs still do that, of course. But the fluff with clerics these days is that the gods teach how to access divine power rather than acting as the conduit. So you could have a cleric turn aside from their god and still keep their powers....by default.
That allows for some stories that wouldn't otherwise make sense. For example, how the King-Priest of Ishtar kept his powers even though the gods were trying to stop him. Other powered heretics are similarly possible under that lore. Though it does the power loss and redemption story arcs (though personally I feel like those work better as fiction storylines than as gameplay ones) and you can replace power-loss and redemption with fugitive/trial/redemption or even remaking the mortal religion of the god.
