When I was 20, I used to be a total sub bottom. I couldn't fuck a guy arse, and everytime I tried I blew my load after a few seconds. But it changed when I met a really nice guy who was really patient and a wonderfull bottom, with a round bubble butt. He mad me fuck him and as he understood that I was too easily discharging, he always asked me to go slowly, to stop a while, then go back again. So I kind of learned to detect the moment when my excitation level was going out of control (the orgasm peak) just to avoid it, staying in the "plateau phase" and stop moving to come back to the arousal phase (it is classicaly considered that there are three excitation phases in orgasm : arousal, plateau, peak). While you enter the peak phase, it's almost impossible not to ejaculate (except if you train reaaaaally hard or were given that incredible gift of being multiorgasmic by your fairy Godmother).
That guy also showed me how I could control a bit my erection by working my muscles. He began by pressing on the spot between my balls and ass where I could feel the pubococcygeus (PC) muscle (the pelvic floor muscle), and made me contract and relax it. I did it a few times a week while stroking my cock during a few months. And this little exercise really improved my ability to restrain from ejaculating.
So basically, I think that working on "not rushing to the peak phase of orgasm" by just letting the excitation going to the "plateau phase" (2nd phase of excitation, the one just before orgasm) and then coming back to the "arrousal phase" (by stopping the movements) and also gaining control of the PC muscle by practicing the kind of exercise I described are the two best recommandations I could make, apart from the psychological aspect (which is even more important) of being really patient with your friend.
