Has anything been changed on the bike recently? A new chain can slip briefly on worn front or rear sprockets when there is a lot of power being put into the pedals.
I'd think the bike shop would know if a crankarm was loose for most all crank types. However if this is a cottered crankarm, then can imagine it might slip but still seem tight on the spindle when examined with no loads. But I can't imagine that the crank arms will still be 180º apart after than happened.
Messed up freewheel that disengages I've had plenty of times before with my kids bikes when left out in the rain to rust. But liberally spraying a light lube, WD-40 or some penetrating oil into the crack between the freewheel cogs and body usually will eventually fix that. But maybe only temporarily.