If they're on Find My Friends, they had to consent to that proactively. The dog's AirTag is less accurate or constant than what they're providing already.

My whole family have this sharing on Google Maps. It's becoming more common.

Consent isn't universal or eternal. Without commenting on the relationship the commenter has with their family, in general, whenever you give someone something that leaks information, it's worth letting them know and allowing them to make a choice.

The opt-in party can withdraw at any time. Here's more information about how it works: https://support.apple.com/en-us/HT201087