There is not an absolute fixed guideline as to what constitutes good naming standards in a given language, except that within an organization and a code base, consistency is king.
There are published guidelines that many have adopted, but these are by no means fast-and-hard. My organization uses C# exclusively, and we_use_underscores to great effect. Style is arbitrary; as long as we are consistent our code is not a mess. (It should also be noted, in light of the ReSharper recommendations in other answers, that we are avid ReSharper users, and this in no way forces us to use pascal case or any fixed style -- it is fully configurable.)
Clearly, your organization lacks a style guide. You don't need to single your colleague out, but you can collectively, with a group of colleagues, broach the subject of naming conventions by pointing out the inconsistencies in your code base. You can collectively debate the alternatives and come up with a style guide for your code. From that point, everyone is bound by that guide, and there can be no offense taken if someone is asked to clean up their code according to the guidelines.