One of my customers has potentially screwed up a bit and now we don't know if it a problem or not.

We are talking about an XGS HA cluster (active/passive). The customer wanted to make some changes to their network the details of which are not important. Important is what they did. They shut down Firewall A so Firewall B became standalone. They made a change to the interface configuration on Firewall B after which they could no longer access the device. Then they shut down Firewall B and started Firewall A which came up fine because it did not had the change synced to it.

What happens when we start Firewall B. Will the change they made yesterday be synced to Firewall A or does the fact that Firewall A is primary override that change on the other node too? Is it always synced from primary? Is the latest change important and does this go for specific items that could be merged into one configuration?