Have we erred and wronged this shaykh by judging him to be deviant and shunning him, given that he sends flirtatious messages and explicit pictures and orders the messages to be deleted, and that we did not expose him and the matter remained a secret?

You both must advise this person and clarify the impermissibility of his action, and that this is uglier coming from one who associates himself with knowledge, and invite him to repentance. If he repents and becomes upright, then do not abandon him. But if he insists on his abominable acts, then it is permissible to abandon him, especially if there is a benefit in that for deterring and rebuking him.