In-Game: If the character is intelligent and wise (which it sounds like he is), then of course not. He doesn't have a strong enough allegiance to Vecna to make it worth it. The power might be tempting, but I think he could see it's not worth it, as he won't really get to use the power unless the Eye lets him.
Out-Of-Game: If he wants to give you some proactive control over his character so you can play up complications, then sure, go for it. If he wants to keep more of that on himself, with you just reacting to the stuff he does in-game, then no, he shouldn't.