Is it okay to leave a short-term job off your resume?

My sis is looking for a new job, and has a pretty solid professional resume. She had one short-term (under 3 months) job that she would prefer not to include on her resume (she was let go and feels they might give her a negative recommendation since her ex-boss had some hostility toward her -- yes, she knows most companies won't do this for legal reasons, but she doesn't want to take that chance).

Obviously if they do a background check, they can find out she worked there briefly, but my feeling is that as long as she doesn't lie if asked, she's not being dishonest leaving it off. Because her resume lists jobs by years, not months, and because it was a short-term thing, it doesn't create an obvious gap on her resume. And in a market where it's tough to get jobs, she doesn't want to have to be explaining negatives on a first interview.

I don't think it's a big deal to leave it off; to me, a job you only had for a few months isn't really relevant to your professional growth and development, especially since she's had other long-term jobs that show her stability and professional expertise. But she doesn't want to lose a potential job because someone does a background check and finds this job and thinks she was being dishonest by not listing it.

Anyone have any thoughts on this? Any HR managers out there who can say if leaving a short-term job off your resume would be grounds for not hiring someone? If asked, she wouldn't deny it, but she would rather not have to put it out there.

Thanks!