Ear pieces: Littman steths have a shape to the thing that angles it toward the adult eardrum a weensy bit better, plus soft eartips help seal other sounds out. However, I have occasionally noticed my own pulse when I have a bad cold. So have her clean her ears out once a week (not more often) with those oily drops you can get from the drugstore, rinsing with warm water. I used to do that in school the night before clinicals.
And yes, to echo the poster who said not to put your thumb on the head of the steth. Also don't let the rubber cord things flap around while listening, they can hit the steth and make little bumpy sounds that could fool ya.
Where is the steth head placed? Feel for the brachial artery and where you feel the strongest pulse, stick the steth head right there.
Practice, practice, practice. You can help with this. You set the bp cuff up on your girlfriend's arm, let her have the steth, you hold the thing where she can see the dial and help her take her own, so she knows what it sounds like. Then have her take yours, so she can hear the difference. Let her try on both arms, and forearms too! With forearms the steth goes over the inner wrist. Works just fine, and good to know for obese people when you can't find one to fit the upper arm.
Also know that if the whole pulse is thready, the loud "bump" that's the first karotkoff sound will be barely audible. If the pulse is good or bounding, there will be no mistaking that first sound.
How is your girlfriend at listening to apical pulses? Does she hear sounds fine on the chest wall? If not, perhaps she's got a hearing problem. Electronic steths, or ultrascopes if it's very mild, are wonderful. I've got an electronic and wouldn't have made it past my first quarter without one. Good luck!