Cyonix said:
It's a small to medium network with about 150 - 250 users. It has 2 domain controllers, both with dns. One of these domain controllers has an external dns server added to its NIC secondary dns servers as well as the two internal dns servers.

My complaint to the network manager was that the external dns server could cause replication issues if the domain controller for some reason falls back to this external dns server.

The more senior tech says that this is standard practice. To me this sounds very odd as Active Directory relies on dns to know who to talk to, so if the domain controller falls back to this external dns server the network is going to stop working.

note, the external dns server is the ISP's dns server.
If the actual Domain Controller falls back to it's secondary DNS, then Active Directory is down (assuming you're using AD-integrated zones, and there is no really good reason not to), so in that case it really isn't an issue (or, rather, you have a more pressing issue to fix!) It's really only on non-DCs that it can be a problem, they shouldn't ever go looking externally.