It is possible, look up pink sleeve on google, it's your best friend on this subject.
Trick your grilfriend into having anal sex, then donkey punch her in the back of the head and quickly pull out. This will force her anus outwards, making it visible.
I honestly hate the way the prolapse tag goes completely misused. Medically this is not prolapse. Prolapse is a very specific rectal disorder where the walls of the rectum have become separate from the body (due to the weakening of ligaments. Therefore sillycon's reference to the wikipedia article is misguided) and a person no longer has control over it. In this instance this woman is simply 'bearing down'. You know, that thing you do when you want to poop, and therefore is making her rectum protrude. It's not the same thing as prolapse. Actual prolapse is far far more profuse.
In either case, prolapse assumes the person does 'not' exhibit control of where their rectal walls protrude (if at all) due to an internal rupture. It also does not assume that some external object is 'tugging at it' and making it visible through its actions. I understand why we 'use' the prolapse tag (because everyone knows what it looks like). I'm just trying to make it clear that realistically this 'isn't' prolapse. For anyone genuinely curious that is and mainly cause I hate when medical literature becomes confused.