I'm not sure what you mean about leading a tune on the uke. Without following the conventional staff notation that sits above it, reading proper uke tabs isn't easy. If your friend is a good and patient learner, he might eventually be able to play a recognisable melody,but even a loud uke playing single strings will easily be drowned out by other instruments if they're all playing the same tune. As a melody instrument, the uke is best when heard solo.

But you say he only plays chords and strums, in which case he just has to follow the melodies that others are playing. To do this, he only needs to be able to play a handful of chords to accompany many tunes. Actually, this could be a much more useful contribution, as the uke's strength in a mixed session is as a rhythm instrument, adding pulse and chords in the same way as a guitar often does. The best chords to start with are those used for tunes in G, D or A. The key of C is an easy one for the uke, but not that common in Irish sessions. If he is happy to strum the chords for now, there are hundreds of uke sites giving chord-diagrams (as opposed to uke-tabs written on the staff) for thousands of songs and tunes; but the best thing is to learn the main chords and then try to play along by ear.

Marje