I recently ordered a Canyon Aeroad and it comes with a “Shimano 105 R7100 4iiii Precision” single sided powermeter which i won’t need because i have my trusty Assioma Duos.
I’d love to swap the cranks and give the 4iii Powermeter cranks to a friend of mine who is currently riding without a powermeter. My questions is whether the crankset would fit his 11sp Ultegra DI2 bike? Could he just run the 4iii crankset with the 11sp chainrings?
As far as I know, all road Hallowtech II parts are interchangeable, so 11 & 12-speed cranks arms/chainrings should work together…
I believe the internal of Shimano chains are all the same regardless of speed and its just the externals that vary to fit in the matching rear cassette. So it won’t matter what speed of crank you have. I have certainly ran in the past a 10sp crank with an 11sp cassette and a 11sp crank on a 10sp cassette.
You can mount the cranks to the rings but it will look funny/incongruent because they aren’t perfect matches. I’m not sure if it would reduce power transfer from the crank to the ring. Seems like I didn’t take a picture when I tried this and I don’t have my equipment readily available.
Are you planning to give him the whole crankset? The 12 speed chainrings/cranks work fine with an 11 speed drivetrain. I’m actually doing that now 
I’ve got 11 speed rings on a 12 speed crankset as a result of getting a new crankset courtesy of the Shimano recall. I only discovered this as I initially couldn’t work out why my Absolute Black oval chainrings wouldn’t fit on the crank
You could just exchange crank arms, if they are the same length. It might look odd, but could/should work. Or swap crank sets?