From the sounds of
@slup report above, it is possibly a battery leaving the aircraft, sometimes they aren't clicked in nicely, or a battery may be swollen, as Lipos do.
As it was new, it shouldn't be swollen, but check other batteries your dad may have, see if they are all nice and flat on that side.
If they are slightly discharged, put them on the charger, nad see if they stay flat, often when warmed up (charging or fling) they swell.
This can make them pop out on the
M2P particularly, there have been numerous cases in
M2P crashes posted on here.
My bet is possibly not clicked in properly on both sides, but the weird thing there is it was in tripod mode, which is slow and gentle.
Batteries popping out is often seen flying in sports mode, or pitching under stress, sudden stops, etc.
The good thing is if it's just an arm, it is a fairly easy repair for most handypersons, and some (fine / micro type) tools.
You and your dad might be able to tackle this together.
Youtube search >
mavic 2 pro (front or rear as applicable) arm replacement < and see what you can find . . . good luck with it, she should fly again with minimal cost.
Even if your dad has Care Refresh, it's probably best to repair such a small problem.
Just check all things like camera are functioning properly before just assuming it's only the arm damage, start it up without props, just use the controls to check gimbal function, record a couple of photos and take a video, check all looks ok.