Posted: 26/07/2016 at 19:05
Many rodos are routinely grafted onto R. ponticum, just as pears are grafted onto a wild quince and apples are grafted onto varied apple rootstocks - an explanation of the whys and wherefores of grafting here https://www.rhs.org.uk/advice/profile?PID=359 . It is perfectly normal and shouldn't be a problem in normal circumstances.
The question is, why has the rootstock put on top growth and flowered (if that is indeed what has happened). Has the graft been improperly performed or subsequently damaged, or has some damage been done to the rootstock below ground, which has stimulated growth?
So that you know what you're looking for, this link should show a picture of the top growth of a named variety, grafted onto a rootstock http://rhododendron.fr/dos23_e1.htm
Look for something like that, and see if there is single branch bearing the pink flowers, and identify where that branch is growing from.