Does she have a woman-specific pack? With her build, she'll need one.

Does her pack have load lifters? I have mine tight enough that my shoulder straps don't even touch the tops of my shoulders, which are extremely pressure sensitive. I've never had any problems with bra straps vs. the pack, but that's me.

It appears from the photo, although at that distance it's really hard to tell, that the sternum strap may be pulled too tight. That would put extra pressure on her collarbones. I rarely even fasten mine, although again that's me! If I do fasten it (in tight places where balance is important), I leave it pretty loose.

It's interesting that in the picture, your (if that's you in front) pack straps fit fairly wide on the shoulders, whereas hers are closer to her neck (and possibly on her collarbones). I wouldn't want my pack straps that far in on my body. Again, that's me. . . .

Can you take more pictures? Front, side, back, all closer up. That would give us a better idea of how the pack fits. It may be she needs a different make or model--pack fit is as individual as shoe fit, and there is no universal fitting pack, just as there's no shoe that fits everyone.

IMHO, she shouldn't have sore shoulders with a 20 lb. pack, if the pack fits properly.