Well, if it's all grain, he's going to have to boil it all. I definitely wouldn't just throw in the extra at the end of the boil. It's incredibly important that you kill any microbes in the wort. Boiling is also important for proper hops utilization.
I can think of two options (not counting buying a bigger kettle):
1. Scale down your recipe to fit your brew kettle. This is simple and can be done with brewing software.
2. Perhaps if you have a second smaller pot, you can put the remaining wort in it and boil it separately. And divide the hops proportionally amongst the two pots.
Whatever you do, boil it all!