I think the MP pipes are larger diameter, so the tips are designed for that diameter, not stock.
He could use these:
http://www.burgertuning.com/exhaust_tips_for_BMW.html
The black, angle cut look great on my stock pipes, w/ the MP Diffuser!
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My 2017 M240i: MHD, xHP, Modded OE Intake, AA GESi 400 CPSI Cat DP, Dinan Mid-pipe, M Performance LSD & Rear Diffuser, BMS Exhaust Tips, Apex EC-7 Wheels, M-Tech Front Lip, iND Painted Mirror Caps, Smoked Reflectors