Basically try to get him to listen to the combination of kick, snare and hi hat pattern for each song on a well produced album. Each song should have a unique groove that should stay consistent all the way through and that’s what his strumming pattern should emulate.
Its not so much about writing the pattern down as he may not know how to, but a competent musician should have the listening skills to pick out something that fits with the groove on the fly.
Our intermediate acoustic DVD package covers that quite extensively with about 35 different strumming patterns and a section on how to find the right pattern in every song. Certainly going trough that course should help a lot, as long as he’s willing to learn.