While your actions may be well intended you have ventured into a grey area. The military services are currently struggling with how to address certain events that occur within the social media realm. These type of situations need to be addressed in concert with the commander and the JAG office. Not at the mid-grade leadership level. This issue may require a policy decision by the command.
I would encourage you to bring this to the attention of your chain of command. It may be necessary to provide them with proof in the form of a screenshot or other similar means.
I would caution you and suggest you do not proceed until your chain of command becomes involved. While I believe your concerns are most likely correct, your actions need to be coordinated through the chain of command. Depending on the details of the situation, his actions maybe punishable under the UCMJ.
I have attached a Social Media Handbook below for your review.