wrote

my in laws just got their chimney swept. the guy told them that the
smoke shelf had holes rusted through it and they should have it fixed
before using it again. Is there any real harm in this situation

Yes. 2 risks. Fire and depending on where, possible smoke damage.
Seiously, do not risk an unsafe fireplace. It's far cheaper to have it
fixed than you might think.

Check the cap on the top. It's a likely spot to have leaked which caused
the damage. Caps arent but about 25$. Get the type that screw in.