For comparison (no e-peen intended) of progression:
Last HST cycle: 15,10,5
Bench, 85, 97.5, 112.5
Squat, 100, 120, 142.5
This Cycle: 12, 8, 4
Bench: 95, 110, 120
Squat: 100, 125, 155/160?
This cycles figures are probably out a bit in comparison to my log as they're off the top of my head. Squats were boxed up until 4's so marginally harder with no stretch reflex which accounts for no notable progression.
I don't think there is always a linear correlation between rep ranges and weights used though a 30KG jump between 15's and 10's does seem like a big jump.
But bear in mind being this being his first HST cycle and his first experience of 15's chances are the 15's weight is lower due to not being conditioned for the strength endurance required rather than miscalculating. If that makes any sense.