Skipping the usual legalese, here's the pragmatic situation:
G&G have no presence in the UK as far as I can see. If they do agree to fund a repair, it will generally be for the parts only, not the labour, which is where most of the cost is.
If the retailer is telling your friend (do they not have the internets?) to GTFO, then there's no magic incantation that you can say to force them to do the repair.
What you can do is assert your friend's right to receive goods that are durable, as per CRA 2015 S9 3 (e), and give the retailer a chance to rectify it. If they don't, then you get it done elsewhere, invoice the original retailer, and start down the long and winding road to a court small claim. You're looking at about a year, with more costs up front, and you might never recover the money. Very few people see these cases to conclusion.
If he bought on a credit card and it cost over £100, that's where I'd start. Tell them that the retailer is being a dick, and cite the Consumer Credit Act 1974 section 75.
All that said, there's no definition of durability. It's just what a reasonable person would consider it to be. I personally consider airsoft guns to be disposable toys, and I'm amazed when they work at all.