Hi BikeGirlfriend, and welcome btw.
Ok then, lets open a can of worms…
IMHO I don’t think that getting a bike fitting for a mountain bike is necessary. The idea “of a perfect position” comes from the world of road cycling where you are covering a lot of miles, but can remain in the same position for a considerable length of time, ie, sitting in the saddle with the head in a tucked position, only changing position when you climb out of the saddle for brief periods. Incorrect bike setup in this situation, ie, seat too high, cranks too long, stem too high/long, can result in injury. In mtb, because you are moving around much more, constantly changing position for climbing, descending, cornering, etc, although the general principles of correct setup are the same (saddle height, brake setup, standover height, stem length) they will not have such a great effect on overall performance. Lots of mountain bikers will already know what the most crucial changes that they need to make are, for example, putting seat down a bit, moving saddle forward on the rails a bit, but these are things that you can do on your own without having to pay someone a lot of money. In reality there is no “perfect position” for a mounatin biker, it’s whatever feels comfortable to you. It’s like asking “what helmet / shoes should I buy ?” There is no right answer, it’s whichever one fits you the best.
I guess the only exception I would make for mtb, would be if you’re racing XC at elite level, in which case small advantages can get big results, but for an average trail rider, no.
Much better would be to get him booked onto a good skills course with a good coach, who would cover bike setup as part of general skills coaching. I can do no better that to recommend Jedi, who is pretty darn good on a bike (also in your part of the world).
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Apologies for going on and getting on my high horse, but I think that it would be money up the wall, and it wouldn’t make the boyfriend a better rider.
TOD