If her leg is stable, which it appears to be then I see where her coach is going. Personally I agree with your friend. In the second picture what I see is a rider with a good base, and allowing her horse to jump but with a too long stirrup causing her hips to unengage and yes, arch her back. Her legs are also a little too far forward for my liking, but this will get fixed when her stirrups are shorter.
In the first picture, I'd like to see her release a little lower, and she is overjumping a tad but otherwise I like it.
I would also like to see this horse put a lot more effort in. It has sloppy knees and is jumping with no bascule. Part of me has the feeling that the reason the girl seems to have just "good" eq is that she hasn't ridden many horses with a lot of movement and jump and just cruises around on this little guy. Her riding would improve a lot with more horse, and I think she could go a lot further in a short period of time with one.