I took 20 years off from riding, when I was last riding it was on a KZ750 that was decked out for touring. After that 20 year break I got my 1st gen, just because it can go 100 MPH in third does not mean that I did and I am still alive with most of my skin still attached. My next door neighbor got a Suzuki Hayabusa as his FIRST EVER bike. That is a seriously fast bike and he is still alive. (I am still working on a chance to take it out for a spin.)
Depending on your friends attitude at a minimum get him to a big empty parking lot to work on basic handling. Better would be to take some courses over again. Since he had you take it for the test ride for him, it sounds like he may be sensible enough to realize his limitations and to accept help and suggestions.
Good luck for the both of you.