Has anything else out of the ordinary happened with this family? Has there always been an underlying feeling of mistrust? You said you didn't "click", but maybe she is just not a very warm and friendly person.
If this was it, I wouldn't even address the issue. If she brings it up again, or has another problem to address, then I would have a conversation saying "I get the feeling you are uneasy about the care So-and-so is receiving. I could be out in left field, but if there are any challenges or things you would like to address, I would like to talk about it to put your mind at ease".
I have had parents question things. But they have all done it with genuine curiosity. One little boy told his mom that someone had been biting him. He was not getting bit, but we did have a biter at the time, that he had witnessed chomping on his friends. I explained that, and that was the end.
Honestly, let it be and go about continuing the quality care that you know you provide. Let mom work out her unease. I would rather have a parent that cares than a parent that doesn't!