At first it seems that a use of the indefinate article before a name is incorrect.
In fact, it may even be said that such a sentence construction is incorrect.
The convention of saying "a Mr Frederick" comes from speach, and is a shortened version of the phrase "a certain Mr. Frederick". Such a sentence is mainly used in speach, but especially in more modern writing, we see such phrases coming into writing.
Simply, this phrase is perfectly acceptable, even though it contradicts rules about the use of the indefinate article (a, an etc.).