Think those lifetime warranties are only covered if he is the original purchaser of the system/exhaust back box.
Lots of those "lifetime" warranties/guarantees are the same. Eg: Non transferable
He did just say he got his from Mitz who is a trader on here, ...The Cougar Store. Maybe best to give him a PM or phone call to see if he can help.
Link: http://www.350z-uk.c...4-cougar-store/
Website: http://cougarstore.co.uk/
Failing that you could always try a custom exhaust place like "Longlife" for example as they will have plenty of different style tips that they could simply weld onto your Cobra box. Just another alternative.
I realised he said he got them from Mitz but i couldn't find him in the Traders section. I've just found him in the members section though.
Thanks