That workout your doing is good for conditioning, but you wont build up any muscles doing that.
Best way to build muscles is, to do weights....but by doing them RIGHT.
Dont curl, or extend, or kickback or any of those other isolation movements that will slow your boxing game down.
Use whole compound movements that work all of the body at the same time.

There are 4 major exercises= Deadlift, Military press, Bench press Squat.
If you do those twice a week, for 4 sets for 6 reps. You WILL build muscle (as long as your eating properly, but thats a different discussion altogether)
Id say go to the gym with your friend.