I have traveled with both my violin and viola (not at the same time) as a carry on with DL. Admittedly, I haven't done so in a little over a year, but I have never had any issues with either of them and the cases I travel with are hard-sided and about the same size as a mountain dulcimer. They fit nicely in the overhead bins. Although, if the flight is full, you might tell your friend to offer to put it in the closet because it will effectively render one bin useless except for smaller items that can be placed in front of it. I think your friend should have no problems.