So this is literally 20 years ago now (plus or minus a month) we had this conversation, and I'm not sure I 100% understood what he thought even then.
What he said was something like that Islam says that when you're attacked, you should be strong defend yourself; and maybe let the other person know that you're not weak. So I guess he thought that this contradicted what Allah said elsewhere (through Mohammed), and therefore couldn't have been said by Jesus (since Jesus also spoke for Allah); and so must have been added in by someone later.
(To be clear, trying to repeat what I remember he said, not making any claims about what Islam does or doesn't teach.)
The thing is, the whole reason Jesus' statement stands out is because it does contradict our natural ideas about the right way to react. I can understand someone adding in fake stories about miracles or fake claims to be divine; but why would you add in fake teaching that's so counter-intuitive -- and on the face of it, so... pathetic?
His question honestly caught me off-guard; I was expecting him to bring up claims of Jesus' divinity, or his death and resurrection. "Turn the other cheek" is so firmly established in our culture as one template for "how good religious people behave" (a la Ned Flanders from the Simpsons) that it wouldn't have occurred to me that people would consider it "false teaching".
And because I was caught off-guard, I didn't ask him how he thought it came to be that way in the first place. Probably he hadn't thought that far. :-)