How do you ease the mind of the primary caregiver, and help her "let go"?

I have been called to do respite care for a very elderly woman with Dementia who has a live-in caregiver. The caregiver, bless her heart, gives 110% 24/7. The problem with this is, she has unrealistic expectations for the respite caregiver. I was asked a series of questions by the agency I work for, which indicated this live-in caregiver had verbally abused a couple of the other employees who were sent to the house. She's never abused me. She calls me the "top drawer" caregiver, which is very sweet. So I guess my question is, since almost all of you on this forum DO live with the elderly person about whom you post - how would a stranger go about helping you "let go" and relax about your loved one's care? I see the tension in her eyes, and the weariness in the way she moves. She's as burned out as it gets. The client is 95, ambulates with a walker, and is surprisingly strong. She eats well, drinks fairly well, and makes it to the toilet quite a bit of the time. She requires monitoring for safety, monitoring for adequate cleaning up after toileting, and encouragement to move around to stay strong. That's really about it! Obviously there is the need to stimulate her brain by engaging her, and the need to keep her awake during the day so she can sleep at night.
What could a caregiver do/say, in your opinion, that would help you relax about leaving your loved one in their care? I start a round of 8-hour shifts with this lady Sunday. The family is just happy she's cared for, and does zero "checking up" on what is actually done. In my mind; if the woman is at peace and feels safe and comfortable, is totally clean hygenically, well fed and hydrated, and mentally stimulated, I've accomplished my assignment. By the way - this advanced Dementia lady always recognizes me. :-) I love that part. This question is more for the other respite caregivers, as I will not be taking more hours in October.
Ruth