I think the approach that Martin proposes in the comment above is really the most practical:
The best way is to edit it in thoroughly and leave a comment under their post, like I did here
I'm optimistic that our core users can use this "teach a man to fish approach" for most cases.
The editing help (accessible via the golden question mark at the upper right corner of any question or answer textbox, then via "Advanced Help"/"LaTeX")
talks about formatting $\LaTeX$, but it doesn't really go into the mhchem particulars. I think we had talked about getting some more detailed instructions into the editing help, but I think it was decided that we would have a separate page in the site help, which can be found at: https://chemistry.stackexchange.com/help/notation.
We can revisit getting it into the editing help, but I think the above link along with the gentle reminders for new users should do the trick in most cases.