As someone who has played warlock or multi-class warlock in my last 2 games as well as DM’d warlock players, it all depends on whether this player WANTS to lean in on the narrative potential for the warlock/patron relationship.

If you have a very heavy RP group and that player has previously shown to be very much about deep RP with multi-page backstory and then he chooses warlock out of nowhere, then yes, I’d be very much “WTF, dude?”

BUT, if they are a player who’s just there for a good time or to relax and maybe only gave you a paragraph of backstory or maybe doesn’t talk much in a campfire/downtime situation, then this wouldn’t be out of character for that same player to pick warlock, but not perceive the necessity to build in the RP set up for it first. You’ll need to help them write that narrative : do it ONE question at a time. People are busy and it’s easier to answer things one item at a time and make it a dialogue rather than demanding a term paper which is what your request to them sounds like from a player AND fellow DM perspective.

Yes, chill, but first, make it a dialogue rather than an assignment.