That looks like some type of film on the lens not MgF etc.
I have read about some fella in the US who tried all kinds of solvents and chemicals who could not get any type of damage to the coating.
I suggest it is a non reflective polymer coating that has been damaged.
If it was me with an expensive eye pc like this I would dis-assemble the Ep and remove the damaged lense.
Then try to find what type of chemical/solvent that damaged the eye p/c and remove the damaged coating from the glass by rubbing a cloth wetted with the "solvent" until all the coating is removed.
What have you to loose? if you can clean up the damaged lens to a clean class state all you loose is one element loosing a coating to one side. Worth a few percent light loss and having a useable Ep again at no cost.
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1