Go on the premise that the aupair could be a mass murdering psycho and screen on that basis.
There are no assurances, even if you use an agency -which are not usually worth their fees the little amount of checking they do.
Your friend has to do all the hard work emailing the girls and keeping up a correspondence, asking references, checking up on references (I look for referees who can speak English and make sure I call them if possible). I check their passport when they arrive, particularly VISA-wise.
The aupair must have a plausible profile, usually gap year student. And their email responses must tally with their profile.
I figure that if a girl is prepared to answer 30 over questions in an email in English, she would probably be interested in the role. An axe murderer would find some easier fish to fry!
I try to stick with aupairs from the EU, so as to avoid VISA issues. It would almost impossible for a person from South America, China, Indonesia, Philippines to be able to satisfy the visa requirements, so I bin those applicants immediately.
'Tis a bit of a minefield. Read the other aupair threads in archive to get more tips.