How to allow user to update only workflow_state field

I have a Document, which has one standard field and workflow_state field associated with a workflow. One role i.e. Role1 has create, read, write permission to the document while Role2 has Read permission only. In the first transition Role1 action will be “Send To Role2”, in the next transition Role2 action will be “Read The Document”. However, problem is workflow_state change is not happening, because Role2 does not have write permission (because I do not want to allow Role2 to change the document standard fields.

How to achieve this?