It's legal, and it's been known for ages, and my guess is that your wizard player didn't spot it, but rather read about it elsewhere.
The solution is to ask him to not be an arse. This is one of those combos that destroys the game.
This is one of those combos... wow...
It can be used once, maybe twice, per day. It is not a game breaker unless the DM literally only uses 1 encounter per day, and then, only if there is a single vastly important entity involved in that fight. If the DM builds encounters like I do, then usually there are no "OMG! Kill it!!" creatures involved. Instead, there is a team of enemies working together to end the PC's lives, or capture them at least.
Also, this tactic does absolutely nothing when the only encounters the PC's face in a given week is social encounters, lol... The game isn't always about combat.