The whole process is probably very human-survivable.
The very end of the hover-slam is about 2g acceleration (dry mass of stage ~23 tons, single Merlin 1D rated at 723kN, throttled down to 70%, so 723 x 0.7 / 23 = 22m/s2 assuming it lands bone-dry).
The boost-back is done on three engines but with some fuel remaining in the tanks. Our user Hobbes computes that given the speed at separation and the duration of the boostback burn that it exceeds 4.6g acceleration; it's potentially as much as 8g. That would make it the roughest part, but comparable to the ascent forces of Mercury or Gemini manned flights.
Aerodynamic braking force will be much less than it would for an orbital reentry (because of the lower speed); at a guess it would contribute less than 1g of force to the experience.
I would also guess that the jolt at touchdown is substantially less than that of an Apollo-style space capsule splashdown, which could be pretty rough, particularly if the capsule hits a rising wave.