There are a few moving parts when you're trying to show someone something on a uke in a different key. It's not hard. Look at the circle of fifths:
Each note is a fifth higher than the one to its left.
The gaps between the notes of a ukulele's strings is the same on both instruments, even though the notes on a normal uke are a fifth higher. So to find out the baritone chord equivalent to what you're playing on a soprano, just move one space to the left of the chord you're trying to play. For example, a C chord on a baritone is the same shape as an F on a soprano.
This is really much harder to explain than it is to do, but honestly I think it is just unnecessary complication when you're starting out. Your neighbour could get some GCEA strings for his baritone, or buy a uke with standard tuning.
