It's probably wisest to encourage the writer to focus on the critic's contribution to the process of revision instead of evaluating the critic. Some teachers find it useful to ask students to write a self-evaluation of their work as critics, especially if they have been part of a group or working with one partner for the whole semester. Remember too that not every assignment needs to be graded. How to facilitate peer critiquing There are various options for group size. Some teachers find it best to have students work in pairs, but others find revision discussion groups more useful.
Group size depends on the length of the class period and the length of the assignment.
It would be difficult, for example, for a group of more than four or five to read and discuss each member's draft in a single class period. Asking students to work in pairs can be an efficient approach, but you will run into problems if there is an odd number of students in the class or if one member of the pair fails to show up on the day you've set aside for in-class work. Some teachers ask students to work in pairs or groups on an ad hoc basis, allowing the makeup of groups to vary from assignment to assignment. Other teachers assign partners or group membership for a whole semester. There are advantages each approach.