Does your company do exit interviews?
Objectivley its either an arsehole manager or a clique of arseholes the company wants rid of, a fight suggests its not to Cliquey. Exit interviews would nail the manager, but if your just escorted out then the company seems a bit sub standard anyway.
Gardening leave is a kind of retainer in the hope his contacts go a bit cold. His old company hope clients adapt to the new bloke dealing with them. He should see if the new company will pay a golden parachute fee to start early if he can steal some or all the clients.