Are you taking 9 speed here, cos anything later, nothing matches between road and mtb IIRC
11 speed. Cassette seems to be compatible, getting that on the bike was the easy bit.
So MTB derailleur is not an option? Will check out derailleur hanger extenders.
[Edit - think the answer is to get the customer to send a photo of the rear setup on their bike at home]
@anthonyj started
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Are you taking 9 speed here, cos anything later, nothing matches between road and mtb IIRC