Will one with record of unfinished PhD on his/her CV be doubted of his/her ability? A PhD student of my advisor (I am a master student), having been enrolled the doctorate program for less than a year, is now unsure about whether or not to finish it.

One of his concerns is that if he continues, but eventually quits the doctorate program, and applies for a normal job, the future interviewer, during interview, may suspect his lack of ability due to the record of an unfinished PhD program.

However, if he does not write on his CV about the years during PhD program, the interviewer may further question about the absent years.

So, does a record of an unfinished PhD program makes employers doubt a person's ability? Or are there other negative consequences in it (besides, of course, the opportunity cost of the time interval)?

In addition to helping me reassure him, such information also helps me to decide whether to get a PhD in the future.
(Not being anglophone, if I misused some terms in English please correct me.)



Edit: Wow, to my surprise, this is probably the most popular question among SE questions I have asked (in all sites). I appreciate every answer and have read all of them, but chose Mr Romik for being the earliest among all good answers.
does a record of an unfinished PhD program makes employers doubt a person's ability?


I very much wish it were not so, but the honest answer is yes. Employers, within the restricted context of reviewing job applications, are forced to rely on very partial and superficial information given to them in CVs and job applications. Moreover, they are given huge numbers of applications to select from under time pressure. They respond to this in a way that I think is completely rational, but nonetheless extremely frustrating (to them as well as to the candidates), which is to assign a highly inflated significance to each piece of information given to them: positive things appear much more positive than they should, and negative things appear much more negative than they should. 

In particular, both an unfinished PhD and a long period of seeming inactivity in one's CV are likely to be perceived at least somewhat negatively, certainly if they are unaccompanied by an explanation, and possibly (to a smaller extent) even when an explanation is provided. It wouldn't kill someone's chances of getting a job, and can be overcome by many other positive things one has to say about themselves, but I'm pretty sure it will have at least a small negative effect.