this is a toughy, for there are so many outstanding episodes. if she's never given BtVS a shot, i would say show her episodes from early on in the series, so that you don't spoil anything for your friend. there are definately awesome episodes in s2 (which is when the writing really started to pick up and find its niche). i would definatley go with Innocence & Becoming pt.2, then throw a comical episode at her, like Earshot. if she doesn't get into the show by then, then it doesn't look like she'll ever get into it. i wouldn't really recommend showing s1 to her just yet until she's gotten into the show, s1 is good, but it is weak considering it's the first one. Innocence & Becoming pt.2 are extremely strong episodes with great writing & acting, they have excellent quirky dialogue and great characters, and Earshot is just so funny (especially with Xander & the jello at the end).
if you show her anything after s3, she's probably going to be pretty confused when she goes back to watch from the beginning, b/c she might be expecting things that won't happen for quite a while. anyways, good luck bringing your friend into the fold. i hope you succeed, b/c she won't be disappointed when she gets through the entire series.