My thoughts - if wheelset or frame broke,or significant component such as stem or handlebar, then I'd agree with pushing for a complete refund.
In terms of this problem, the OP describes a front mech problem that is then traced to the shifter. Both are replaceable parts (indeed, if Campag possibly repairable), and with regards to the delay, that might be Giant's fault,or it might not be - the shop may be dragging heels. Or is it that they are busy in the workshop, and that to turn the job around faster would mean defaulting on other customers work? I would go with the diplomatic solution first, and also contact Giant to let them know about issues with one of their retailers.
In terms of a field repair on the day of the problem (a bit late, I know), then if it was a standard front mech removing the cable would have allowed the mech to spring into the inner position giving lower gears that may have helped the return journey somewhat for your son.