Hmmm - planing a long edge square is quite a challenge for a novice -
especially one that hasn't even used a router before. Hopefully he can
get fully squared up boards that don't require it.

I'd agree, butt join the boards - but ideally with biscuits (which
makes getting the two boards dead level a trivial task). Glue with
cascamite or polyurethane, and sash cramp up.