In my opinion you would have no problems going solo in similar conditions, based on what you said. I hike a lot by myself - and it's not like the Whitney trail is deserted.
However, May can be very rough in terms of snow and ice above trail camp. The hike takes on a whole different complexion - the switchbacks are sometimes impossible to hike, requiring snow climbing straight up the slope, ice axe, etc. I suggest you plan for later in the summer unless you've had snow and ice experience.
For late summer, plan on the point you reached last time being a little less than "halfway there" in terms of time and energy exertion (not in terms of mileage - the mileage halfway point is around trailside meadow). In May, you'd just be getting started at the base of the switchbacks.
By the way, I lost 80 lbs on Atkins a few years ago, and the diet has absolutely done wonders for my energy level on hikes. If your friend is like me, and if he stays with it, he will undergo a transformation that allows him much longer sustained exertion. At least that's what happened to me.