I think shei2 is standard.But in China,shui2 is also ok.I suggest you use the former.If you have any question,you can contact me.

shei is not beijing accent, because I am from Xiamen(south city of China). I say shei and I don't use shui in general. But when I started to learn pinyin, what I was taught was 'shui' and my teacher said 'shei' was incorrect. But both are correct now. And I think most chinese people can understand both two pronounce.

Hi, Jack. 谁shéi is much more vernacular or vulgar than 谁shuéi, but, just as you supposed, most Chinese people know both the pronunciations, so I think that the woman who you talked with and who did not know the shéi pronunciation was just in an occasional confusion. When you have long lived in north China (especially in Beijing), you'll see that people say shéi much much more than shuéi in daily life( in fact almost never say shuéi ) , while professional broadcasters almost never say shéi in (especially news) programs. As a dialect, Mandarin has lots of characteristic aspects different from Putonghua, especially in vocabulary. Owing to their special geographical status, people speaking Beijing or north dialects have more chances to speak their dialects in the broadcast, shunning those too slangy word so as not to be difficult or unacceptible for the audience from other regions. Ordinary people not from the north would likely be unaware of the shéi pronunciation, while knowing the standard pronunciation shuéi of Putonghua; but they'll perceive the dialectal pronunciation with ease.