I set up a test course with the following settings:
Group mode: Seperate Groups
Force: Yes
Multiple groups w/ non-editing facilitators.

Here's what I found:
1) When selecting Quizzes from the Activities block, the non-editing facilitator sees 'View 3 attempts by 3 users' even though there is only one user in his group.
2) When selecting that link he only sees the one attempt by the one user in his group (as he should).
3) Selecting the Detailed Statistics button shows him ALL attempts by ALL users (this is not good). Simple Statistics only shows his group users.
4) Selecting Delete All attempts deletes ALL attempts by ALL users (this is what I discovered the other day in my production course).

#4 is biggest problem!