Nothing is known for sure about Hermodorus.
An article on "The Twelve Tables" in William Smith's A Dictionary of Greek and Roman Antiquities says this:
"It is further said that Hermodorus an Ephesian exile aided the Decemviri in drawing up the Twelve Tables, though his assistance would probably be confined to the interpretation of Greek laws. ... This tradition was confirmed by the fact of a statue having been erected in memory of Hermodorus: but it did not exist in the time of Pliny" (article, dictionary).