I know you know this, but for others who may be reading this, I offer my $.02 worth, and maybe it wil help someone.
I was taught there are 4 causes of water hammer.
1) Valve closing too quickly - Although various designs serve to reduce the intensity, if the velocity thru a valve is too fast, the change in velocity when the valve is closing will instigate a reverse shock wave we refer to as water hammer. One reason the 5 fps rule is encouraged. At any speed, as the valve closes, the velocity increases, which is why the hammer starts after the first half of valve closure. "Dampening" this retro shockwave can be accomplished by the supply side pipe, and is related its size, length, and to some degree elasticity. So depending on the length and size of the pipe enlargement, it might work. So, 6 foot of 6 inch pipe might work, and worth a try. I think the reason it would work is not so much related to the velocity, as to the dissipation of any shock wave, similar to how a shock arrestor works.
Although I have never had to do this, I bet it will work. Maybe replace the 3/4 inch manifold with 1 inch (may not make a difference, but since you are there anyway). Another solution would be in the selection of a slower closing valve.
other causes of WH
2) To rapid filling of your empty downstream pipes. You did not state if the WH occurs upon closing or opening. Your friends solution would not work if this is the cause.
3) Air trapping: I have had this problem in long laterals on slopes. I have upsizesd pipes and used check vlves togeher, and one or the other helped. Perhaps, it was the replacement of the undulating smaller lateral causing the problem that helped. In class, air relief valves were suggested, but have never had to used them. But, again, I don't think your friends solution would help if this is the cause.
4) Reverse flow with pump system (I had to go back to my notes to remember this one). I don't deal with pumps, so I have no idea if your friend's solution would help.In class they mentioned check valves as a solution.
Summary. I think if your hammer follows the closing of the valve, it is due to the shock wave initiated by the incresed speed thru the valve. Upsizing the volume of the supply from a 3/4 inch (assumed, not stated) supply line to a 6 inch, effectively adds a shock arrestor, and will most probably solve the problem. I would rule out the other causes of WH first.
Best answer
Keep velocities below 5 FPS when you design systems
But, as I said when I started, I think you know all this, which is why you have waited a year to ask it again.
Hope this helps, and thanks for the interesting question. Jeff