YABU, I think. It’s a bit of an odd attitude to take.
I think you and your husband are separate people who are only responsible for their own actions, not each other’s, and have independent thoughts, feelings and beliefs, (presumably). Therefore, it is the normal/right thing for you to be treated as individuals in situations such as this- if you had both said or done something, then the “problem” would be between your friend and both of you. She is not upset with you over the incident because you did not say whatever it was that upset her. That’s entirely rational and normal behaviour.
I can’t really say whether she is right to be upset by what your DH said, given the information you’ve given. I suspect you don’t really know either, as you weren’t there. Sometimes it’s not what is said, it’s the way it is said. Or your friend is over-reacting. Who knows which it is?
I do think if your friend has done you a favour and she’s reporting bad behaviour by your DC then the most sensible thing to say would be “sorry about that, I’ll speak to them about it”, or similar- she obviously felt it was significant enough to bring up with your DH when he collected the children. Even if it seemed out of character, I would have been sorry that my children had annoyed/upset my friend who was doing me a favour (even if I wasn’t sure what had happened/whether she was “right” to be annoyed), so I would have said something like “sorry about that”. You & DH could then have clarified from the DC’s perspective what has happened- and if you felt it was not important enough to require the DC to apologise or for them to be chastised then you would just have to recognise you and your friend have different expectations of behaviour and not ask her to look after the DC again/decline if she offers.
Telling her “it doesn’t sound right” is equivalent to suggesting your friend is making it up/over-reacting/got it wrong. As you hadn’t had the chance to even ask the DC about it (and I am always hesitant to take DC’s versions of events as gospel), you had no reason to doubt what she said. That is quite rude, especially when someone has done you a favour by looking after your DC.
I suspect she doesn’t want to discuss it with your DH as she is angry/upset with him abs she has spoken to you about it as she is your friend too. It’s not pitting you against your DH. It’s not binary- both of them can be right and wrong at the same time. For example, you can think your friend over-reacted to something your DH handled badly without having to pick sides. You can understand why your friend is upset- even if you don’t agree that what your DC was “wrong” and think DH was a bit careless/clumsy in his response without thinking your DH was deliberately being horrible.