As long as he displays the symbol of the Shramana (monk), then with this symbol, you cannot revile or despise. Make a note of this point.
The third prescriptive precept is never to revile or despise members of the community, renunciates, or those who merely possess the symbols of a practitioner of virtue [someone who is dressed as a monk or nun but has no vow]. Nor should you do this to the mere symbols [part of the robes] of these practitioners. Never in any way divide yourself and others into opposing factions and view the others as enemies. Always treat members of the community with respect, as though they were the community jewel itself.