A big component of the ubiquitousness of plaques may just be the ease of availability. Twenty years ago it would have been expensive and a pain to get one made up. Today, I bet I could get a cheap but serviceable one in just a day or two.
Per the Guide to Safe Scouting:
The Cub Scout exception does not apply if she does not have a child camping, which means that she "must be currently registered as an adult volunteer." On the surface, it sounds like she would meet that requirement, as she is a registered member of Scouting America. However, I've been advised multiple times by professional Scouters that this rule requires the adult to be registered with the unit they are staying with. Simply being a registered member in another unit doesn't transfer permission to stay overnight with other units. It's possible that your council has chosen to interpret this differently. The most likely reason why this wouldn't be a violation, though, is because it was a council event according to OP's description. That may change things as it's no longer a unit event.
Not sure what the issue might be, other than surprise to you. IF she is properly vetted there should not be a problem, especially if she was not necessary as leadership for your unit at the event.
Sorry if this has been asked and answered before, but I couldn’t find it on the forum. I’m a leader with a pack that recently attended an overnight council event. Our committee chair notified us at the last minute that she would be camping with another pack that she has friends in, rather than our own. She does not have a child in scouting this year, and camped with them on her own. The rest of our pack is a bit confused. Is this normally allowed?