Actually, 140 is relatively fast tempo. The most common tempos are 100-120, but also up to 160 is fairly common in popular music.
Playing in slow tempos is much more harder... It tests your ability to place notes accurately in time, while fast tempos are mainly muscle exercises. When you can play something well in slow tempo you are more likely to play it fast well too.
I guess he is new to music in general. So the fast bleeping of metronome will cause him trouble to recognize what to hear. And to know what to hear is really important. In slower tempo your brain is less stressed by the unknown so you are more able to focus on how off the beat you are. In fast tempo it is harder to understand if you rush or drag... When I started I was told to practise basic rhythmic values (Eight notes, Sixteen notes, Triplets...) in various tempos ranging from 50 up to 100. Later I was told to try even 40 and, of course, tempos over 100 BPM.