If an editor appoints a non-expert reviewer by mistake, is there any way to convey this to the editor?
The question seems to be predicated on two assumptions:

1) If the review was not blind and the OP knew that the reviewer was his Former Coworker, he could make a convincing case to the editor that FC was not qualified to review the paper.

2) Thus, because he suspects that the reviewer was FC, he wants to contact the editor to find out if this was the case.

The second assumption is really a mistaken one, I think: according to the principles of blind review, you shouldn't know the identity of the reviewer.  If you think you do, you should nevertheless act as if you don't, and if you have partial information, you should definitely not go around sleuthing to see if you can solve the case.  This is one of the basic cultural assumptions of peer review.  I think that if the author goes down this road, it is very likely that he will be told that his actions are inappropriate and will burn bridges with the editor and the journal.

I find the first assumption problematic too, in a more subtle way.  The reason that reviews are anonymous is that you should not be engaging in an ad hominem discussion of the reviewer.  Just because you know someone's identity doesn't mean that you are an authority on their academic and intellectual qualifications.  Maybe FC didn't know your subject well back when you knew him, in your opinion.  How do you know what he knows now?  

Choosing a suitable person to be the referee is the editor's call, not the author's.  Calling attention to someone's identity -- even hypothetically, as in "I don't know who the reviewer was, but if it was FC..." -- as a strike against the referee report will be regarded by many as obnoxious.  If you have anything to say to the editor, it is about the referee report itself.  If the referee report contains a comment that through its specific brand of inexpertise suggests to you that it was made by FC, forget about the FC part and explain why the comment is definitively incorrect.  If you can point out clear errors in the referee report that are in the serious to egregious range -- i.e., a reasonable person would worry that they compromise the integrity of the verdict -- then you have a case for getting the report thrown out and/or getting an additional independent report.  More likely the situation is not so clearcut, and in my opinion you should still compose a polite response to the editor if you feel that a lack of expertise played a factor in the report, but you fully expect to resubmit your paper elsewhere.  

Getting a bad (as in, less than skilled) referee report and having to resubmit elsewhere is quite a common feature of peer review.  Luckily there is a lot more than one journal, so you can just start fresh elsewhere, and doing so is usually a much better idea than trying to extract reparations from the editor and/or the referee.