Is it OK to discuss a student's work without them present?
I think the basic rule is that you should not evaluate one student in the presence of others. The problem with carrying out a public evaluation "with the student's consent" is that that the student may feel pressured to give their consent.

On the other hand, if a student gives a talk in a friendly environment, then I believe it is appropriate to hold a public discussion about how the presentation could be improved. Care is needed, but I have found that it works. We usually start by asking the student what they would do differently next time.