Is sending a message containing a fatwa regarding a transgression I witnessed sufficient to fulfill the duty of forbidding evil?

You must forbid him and explain to him the impermissibility of his actions. If you send him a message to that effect, there is no harm. However, if he does not stop, and you are able to prevent him physically without a greater harm resulting, then that becomes obligatory upon you, due to the statement of Ibn al-Arabi: "Indeed, the Prophet, peace and blessings be upon him, began in his explanation with the last action, which is changing the evil with the hand, but one should begin with the tongue; if that is not effective, then with the hand."