How to restrict the user to add comments only to the work item , and not allow to make other changes?

In  our environment we have a precondition plugin , so that Owner of the work item  can only edit and save the work item. Now , we have another requirement that , any logged in user ( may not be owner) can add the comments and save the work item.  In the other words, Any logged in user can ONLY add the comments to the work item and he is NOT suppose to edit any other attributes and save the work item.If he did any other changes (except comments) while saving the work item , error message should display .

Could you please  is there any way to solve ? If so , kindly help me.

Thanks in advance,

Jyothsna.