Is it common for doctoral dissertations from for-profit schools to be unavailable?
I don't know about for-profit universities, but the practice at "good" US universities is generally as follows.  Before being eligible to graduate, the student must submit a copy of the dissertation to the library and to ProQuest.  By default, after being processed, it is then immediately available to the public from both sources.  (In some cases it might not necessarily be available online, or might require a fee, a visit to the university's library, or an inter-library loan request.)  

The student may be able to request an "embargo", in which case access to the dissertation's text is restricted for a period of time.  For example, this could be used if the student intended to commercially publish the dissertation or related material.  At my PhD institution, this period can be up to 2 years; any longer embargo requires the special approval of the graduate dean.  Even in such cases, I would expect that the library and ProQuest would still show the dissertation in their catalogs; they just wouldn't allow access to the text.

In your case, "embargo" sounds like the only possibly legitimate explanation. But if the student graduated several years ago, or if the university library and/or granting department cannot confirm the dissertation's existence, I would declare shenanigans.  (Maybe NSFW).