In practice, this doesn't happen nearly as often as you might like to believe. Where I work, we monkey patch occasionally, but never the standard functions. It's usually a 3rd party library that we're editing that we need to bend to suit our use case.
Is your shop 20+ developers? Monkey patching isn't dangerous in and of itself, to my mind, but the prospect of code-surprises is ever present. When I read code, I may not realize it has been overwritten elsewhere. I'm not sure at what number of hires that guy makes his way into the group, but debugging becomes an increasing part of development costs.
