His new 4.88 gears pull him down the slickrock at an unsafe speed? He must have explained it to you so what he said was misunderstood. Regearing from a higher ratio like 3.73 to a lower ratio like 4.88 (4.88 is a lower ratio than 3.73 in ratio-speak) slows the vehicle down, it does not speed it up.
In other words, you would descend or ascend slower
with everything else being the same with 4.88 gearing than you would with something like 3.07, 3.73, 4.56, etc..
So unless he considers a slower speed to be more unsafe, someone has it backwards.