If he's still in contract, there is a warranty on the router.
If he's out of contract, threaten to leave and you'll get one free.
When calling its advisable to know a few details of the package your going to eg offers available so to the agent it looks more like you actually intend to go.
I'm sure you can go to talktalk and get cashback via somewhere like quidco or topcashback, get a free router and a few months free or 1/2 price. So a "free" router from sky isn't exactly a lot to ask for in exchange for continued custom.
if the initial agent won't offer you a free router, ask to talk to retentions.
Sky Fibre Unlimited
Ashington (Northumberland) Exchange