A guy with that height needs to find a bike that fits him.
I know someone close to that height. He went out and bought a walmart bike. I have never seen anything so sad in my life (well, on a bike anyway) it clearly did not fit him and it was not very comfortable!

I don't know how you're going to be able to get away with buying him a bike unseen. I hope you figure out how to get him on a bike that fits him so the two of you can ride together, good luck!