Are you sure this is the right place for such kind of check?

I think it should rather be somewhere in llviewermessage.cpp (maybe process_improved_im()).

please add:
* a reference to the ticket
* a space after //

I'm not quite getting why there are two code paths for calling inventory_offer_mute_callback(), i.e. why not write it like this:

===
LLUUID sender_id;

if (mFromObject)
{
	const LLUUID& object_id = notification_ptr->getPayload()["object_id"].asUUID();
	sender_id = object_id;
}
else
{
	sender_id = mFromID;
}

gCacheName->get(sender_id, mFromGroup, boost::bind(&inventory_offer_mute_callback,_1,_2,_3,notification_ptr->getResponderPtr()));
===

And please move the comments to the lines where the corresponding conditions are.