Hello and welcome! A lot depends on which arm she broke, and where it is broken, along with how far from the time of injury to time of travel.
I had some serious shoulder issues for an extended time, and took some trips to DL and WDW during that period. For much of the time my arm was in a sling, which does get very hot in the summer (and winter!). I recommend that if she does have a sling, that she get the really good quality support ones as they do make a difference. Also making sure that she experiments with shirts etc to find ones which don't rub.
If she is flying, she should try and sit on the side of the plane which is most comfortable for her - again depending on which arm is injured. Many airlines don't have pillows anymore, so she may want to bring one for support.
As to rides, etc - depends again on how she is doing and what she injured. For the most part, I was ok to ride and only holding on with one arm (but I don't do RnR and rarely do Space Mountain anymore)
For me, the bigger issue was just the weight of carrying around dead weight all day. One doesn't realize how much a limb weighs until one has to carry it around for long periods.
Hope that helps, and that she still manages to have a great trip!