I call it bud blast, but it can be either of two things:

Bud blast is a symptom of the fungal disease Seifertia azaleae, but may also be caused by a late frost or application of a nitrogen containing fertilizer too late in the season which prevents the buds from hardening off. In all cases the buds turn brown or black. If it is a Seifertia azaleae (or Pycnostysanus or Briosia azaleaea) infection, the dead bud will be covered with short hair-like structures. Seifertia azaleae is best controlled by sanitation, but may be reduced by spraying buds with fungicide. Avoid application of nitrogen after late spring, but phosphorus and potassium may be applied during the growing season.

I suspect that the warm weather caused the plant to break dormancy, and then the frigid weather froze the buds that started to open. Different varieties have different resistance to breaking dormancy. This one apparently broke dormancy fairly easily. It may be prone to fall blooming also, which is never satisfactory.

Here is a link that might be useful: problems with rhododendron flowers

I believe the photograph shows damaged leaf buds rather than flower buds. Since the fungal diseases which are associated with bud blast are primarily infections of flower buds, I think rhodyman's second diagnosis of a late and lethal frost is the correct one, even though it is odd that only one plant out of 70 was so damaged.

As for what to do, I'd prune just below the current damaged buds and hope a)secondary leaf buds emerge and b) they have time to harden off before winter.