We hire this new guy, a senior-level guy (all we needed was a mid- to junior-level guy to do some work) and he's rewriting fundamental architecture in the code and database. The other developer (junior-level) just fell in line and did it the way he wanted, but it's interrupted my work and now it's broken in staging. I'm having a hard time convincing our managers that this isn't a good way to go about things and the new guy insists that we have to do it this way or it's "wrong" or "poor" and he's even admitted he won't work with a big chunk of our architecture and is just writing his own way of doing it.

Has anyone else encountered this and what was done about it? What is standard practice?