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I would also replace the stock Y-pipe with a hi-flow unit. But that's not your choice, so this should work.
Cut the muffler off just at the Y-pipe end. Then carefully slit along the length of muffler pipe that remains on the Y-pipe (a cordless Dremel works great here). Then pry that slit section open and remove it from the end of the Y-pipe.
Well, he can still use an aftermarket rear pipe, even if he didn't opt for the 2 1/2" system. The mail order guys should have a direct replacement y for under $100.00. If you use the old y-pipe, you are going to have a bit of a time aligning those tips, it's tough enough with the complete system.