Its up to you man. It never ends what you determine best. I've used 22-20, 22-21, 22-19,18,17
They all have their perks. But it depends on what you feel. I ride a long pure, stretched out cockpit, long cranks, and 22-18 gearing. I like it because I feel I can control my power a bit better. Its so nice for street which is my main trials atmosphere. Yet I can still ride natural, however easier gearing for natty is beter imo.
Just try everything... Bars, stem geo, frames, cranks, gearing... Find what suites your riding. Because as your trials career goes on, your riding will evolve from one style to another and certain geometry set ups will work better for each.
Experiment.
As for a good medium gearing, I'd say 22-19 is a very neautral gearing good for street and natty.
It might be a good idea to start on a lower ratio and progressively move higher as u go bigger. I am guessing he can't experiment. Probably has to buy a fixed gear and doesn't want to buy 4 or 5. I think 22/19 is the most generic ratio.