This type of branch collapse (dieback, with wilting foliage) is unfortunately quite common for Rhododendron, and can be hard to diagnose with precision as several fungal diseases can cause similar-looking symptoms. If the plant was drought-stressed, one possible culprit is Botryosphaeria canker, which tends to take advantage of plants that get too dry or which are subjected to high heat. If the plant was over-watered or otherwise stayed too wet (or if the soil didn't drain well enough), then Phytophthora or a similar fungus-like pathogen could have killed either roots or branches. (Root dieback can also result in branch dieback.)

There is no treatment or cure for such infections, and all branches with off-color or wilting foliage would need to be pruned off (don't compost them). Fungicide will not provide any benefit, and if you suspect it got too dry, monitor the plant for watering needs by feeling the soil about five inches deep. If the soil is somewhat dry the touch at that depth, water thoroughly, but if damp when checked instead, watering should not be needed. Foliage can wilt for plants that are both too dry and too wet, so by itself isn't a reliable indicator of watering needs.

Miri