What’s the best way for a 5’8 in 150 lb guy to restrain a 6’5” 225lb violent drugged up criminal ?

Shoot em in the leg. . . . Or the head. Either one works.

Most drugged up people end up weighing 115lbs

That is why the police use partners and “swarming force” to overwhelm with a. Show of such force that the criminal knows they have no way to fight just surrender.

This is one of the key factors of the movie robocop to show how broken and ineffective the police force was in that movie world. When police had to go in outnumbered to try to face down violent criminals with no hope of backup it was a huge statement.

Stun gun, a net, low-level attack on the legs to a painful bar arm, while sitting on and controlling the ankle.

Then some well-deserved punches to the ribs.

Kick the balls....

Tell your friend to stop buying drugs from him. 

There are techniques.  Places you can hit a big man. But Id say the best way is to stay off him after you hit it with your best shot. The knees are a good place to start. Most men  have weak bellies. Hit him there and weaken the knees.

edit: Thats very true. But if you hit him hard enough, it will give you a chance to get away. So hit him again in the knees....

From 100 yards away, use a rifle.

Take him down by any means necessary, restrain him, and toss him in the back seat. 

Taser, when properly used, are quite effective, and not deadly in most cases.