First, I told her that her English was very clear and easy to understand and that I was really impressed with the depth of her detailed knowledge about all the old towns, castles, churches and other places we visited. (I gave honest, positive feedback.)  I told her that I was a speech therapist specializing in accent modification. (I established my credentials).  Then I asked her if I could correct some of her her pronunciation and grammar since she had told the group that she was still working to improve her English even more. She said, "Of course." (I asked permission to give corrective feedback.) In a private conversation, I gave her immediate feedback only for a few words like "focus" since she said "uh" instead of the sliding, rounded American "oh" and I'll let you imagine what it sounded like she was saying!!  Then I said I'd like to write down other features with corrections in my small notepad and that I would organize my notes and give her one piece of paper at the end of the tour. (I kept the feedback confidential and didn't correct her in front of everyone).  She was very happy with this arrangement.