Agree with above. The fact that you call it a 'wellbeing' meeting, which is so right on with the HR brigade, and use the term 'AWOL' suggests your company has proper HR policies and procedures and these must be followed, including how to manage these meetings. Prove when this staff member was told the rules on social media use, were they told the effects of going AWOL? drmorgans is right, make sure you don't incur a risk of legal action.
And I'm afraid you've not been officially allowed to take people down a peg in the workplace since the late 70s :-)