Are friends' sitters always off limits? I think about sitters as folks who are generally looking to earn a bit of extra income by providing childcare. If the sitter has additional hours available that a friend is not using, isn't it OK to fill up that time? I like to think that a sitter who can meet her extra income goals is a more stable sitter which is why I generally don't mind sharing my sitters with friends. Yes, there can be the occasional scheduling conflict but, overall, it seems to work out well.
Is it better to approach the friend or sitter about some extra care time for your family? Again, I generally consider sitters to be free agents so that I can speak with them directly. However, if I know the sitter's schedule with the other family, I'll make a point about not asking for that time (for example, it's OK to ask a day sitter for some extra time in the evenings but not to ask an evening sitter for every Friday night when she's committed to another family).
What about the sitter who is looking for "a new situation"? That can be really tough (you don't want to end up ducking around corners to avoid your friend). In that case, I generally don't want to be the one to "break-up" the relationship.
All in all, I consider sitters to be free agents above and beyond any long-term commitments that they might have. It's OK to ask them for child care when they don't have any other obligations and, as a friend, I'm comfortable recommending my sitters in that regard as well.