The ethics of sickies... what would you do?

I'm firmly of the view that personal leave / sick days should be used to look after your mental health as well as your physical health. Perhaps the employee taking the "sickie" needed a sporadic day off as they were struggling with anxiety or depression which may not be outwardly visible. The friend they were meeting may have been a trusted person to discuss their issues with. In my view, judgement of the employee, who has been identified as a high performer adds no value here.

A previous employer of mine gave all staff a yearly "Doona day" which they could use at their discretion when they simply "couldn't be bothered getting up"!

However to use what is designated Sick Leave for what would otherwise be an Annual Leave day, is unethical in my opinion. This is an abuse of trust.

I think you need to take your other colleagues into consideration. Being absent from a team means others need to pick up the slack. Of course, if you're unwell your team will cover for you but should they have to if your 'taking a sickie' when you don't need to? If my colleague did that, I'd cover them once or twice but after that I'd have to say something and if they continued, sorry but I'd let them hang.

What of the trust between management and their teams? Is it worth risking that? I wonder if the 'sickie' takers are the same that raid the stationery cupboard? And then of course, what happens when you're really sick...?

Probably depends on the context, reason for sick leave and apparent patterns of the employee.

If the employee is trustworthy and high performing, I would accept as a “once off” but still relay that under normal “sick leave” it’s best not to relay social outings on public forums as it undermines their broader trust personally and professionally.

Agree with prior comment, mental health leave is still relatively taboo and if your staff genuinely need a day to reset or assist with diffusing a stressful environment, that’s ok. It is then just balancing the work at hand and ensuring your focus your efforts to coach / work through underlying issues causing the stress.

I'd have a chat with my colleague Jacques but I'd make it non-confrontational.
Some thoughts...
We can't make any assumptions here, just because he doesn't look sick, it doesn't mean that he is not suffering from poor mental health or otherwise. Perhaps a good way to start is to simply ask, "Are you okay?"
Jacques will obviously need context as to why you're asking so you could bring up the social media post.
Or, you could find a way to slip into a convo, "Hey, I was on Facebook last night and you popped up in my newsfeed..."
The scenario doesn't above confirm whether Jacques or the mutual friend created the post. Assuming Jacques didn't make the post, I'd ask him if he knew about it and/or what the photo caption implied. I'd suggest to be careful with social media because other colleagues or his employer may be able to view and may make assumptions without knowing full context.
If we don't ask Jacques if he's okay but instead just talk about the post, the process may allow Jacques to explain the circumstances but he is in no way obliged to - remember you're a colleague, not his boss.
It could well have been a mental health day and the process of catching up with an old friend could've been a great exercise in self-care.
If Jacques discloses that he "pulled a sickie" and was not genuinely sick, we can't assume that he is not just saying this as he may not feel comfortable discussing his circumstances with you.
Effectively, this is a convo between you and your colleague, it does not need to go any further than that even if Jacques admits to "pulling a sickie". This is the first time you've seen this and you've raised it with Jacques in a non-judgmental way, giving him the opportunity to elaborate should he wish.