Try these steps...
- Do your best to make an informative and helpful comment to the OP;
- Then use Mjölnir on it so that it becomes a duplicate (like somebody is supposed to do when the situation calls for it)
The comment disappears! It's gone and cannot be retrieved. So the choices are to write the whole comment again or to forget it and move on.
Is it a bug? Surely not, but what's the rationale for deleting the person's comment?