It's a terrible problem, but one, I'm afraid, that is not easily solved by the mic's owner. The foam, once liquified or getting gooey in the process of deterioration is very hard to remove from a diaphragm/membrane, because the same solvents that would remove it will also dissolve the diaphragm.
ElectroVoice mics like the RE 20 are notorious candidates for having this problem, and their inner foam construction around the capsule needs to be replaced every time such a mic goes in for service.