If it’s added as “Dead on Arrival” then O2 would have to give him a replacement right there. If it was classed as breaking after arrival due to a manufacturing defect then yep it’s up to O2 to do their repair process which sounds ridiculously long, but normal for phone companies. He would essentially be not accepting the phone, rather than accepting and returning.

The phone companies might try to push him into the return process so that they can sell the phone that would have replaced it to another customer, but they aren’t supposed to, and will change their mind when you push back and tell them your rights.

This is one of the reasons why I only buy my Apple products from Apple, their service is far better than this, although has declined in recent years.