Letting him out isn't such a bad idea, but keeping him in the garden could be a big problem. He should grow out of it and neutering will also help.
My only advice for this is to clip his claws.
If your friend is in anyway inexperienced at this take him to the vet. If you don't know what your doing you could cause him alot of pain.
Also, if you decide to do this and he's as bad as you say, then make sure you put on a big jumper and gloves, this will also involve 2 or more people to hold him. I have to do this with my parents cat Frodo and he hates it. My own cat Simba (in contrast) sits happily on the floor and my partner can clip her claws without issue.
Once you know what your doing try to get it done as quickly as you can, but making sure not to hurt him. Aleast him attacking people won't hurt so much and he won't be able to climb stuff, thus saving the furniture.
Hope this helps.

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Simba9952

Through a Cat's eye, all things belong to the Cat !