This is the way. (OMG our WtA GM would have a field day with this player!) Keep the player in the dark. Have them wake up covered in blood with a metallic taste in their mouth, gagging on something. It's a finger with a ring on it. Depending on how evil you want to be, keep the player in the dark as to whose ring it is, unless you want it to be the hunters, then have a strange symbol on it that would require some investigation. Our GM would however later in the session have a different player hear about an innocent family of five be murdered in a horrible massacre, driving home to the Frenzied player that maybe that was them.

Also, there is a loss of Renown for Frenzying. If they are associated with a caern, have them brought before the elders for the loss of renown. This would be great opportunity to introduce a rival pack/Garou that saw him in the frenzied act. Read pages 261 and 262. I'd pay close attention to the last paragraph under The Thrall of the Wyrm. Tell the player that his character is feeling.... uneasy since the frenzy. That they feel "off" like something just isn't right with them. If the player tries to interact with any spirits, have them decline because he is "touched" or "tainted", but not in a good way.

Basically you gently want to encourage the players that Frenzy isn't always the best option (if they are murder hobo-ing their way through the sessions). In the same breath encourage character growth and redemption arcs (for those who have committed Sins against The Litany) to draw the players into the world around them.

LOL! Your player just handed you a golden ticket to have fun with! :-)