Has the child started school yet? I would expect that the teachers will begin broaching this subject, if they haven't already.
If she's not in school, and it were me, I would bring it up, but as Barbara said, in a gentle non-judgmental way. I certainly would not *name* what I'm thinking -- in other words, I wouldn't mention Asperger's. I would probably just comment that she seems to struggle with social interactions, reading social cues, etc. I typically let parents know that the 'worst' that can happen is that they find out there IS something going on with their child, in which case, they've found out at an early age and can begin educating themselves AND teachers (which I believe is a HUGE piece, becasue Self-esteem will depend largely on how the child is perceived by teachers and peers). The 'best' that could happen is that you find out there is *nothing* going on, in which case, if you used a good evaluator, you'll still have some excellent information about how your child learns best, what her strengths and weaknesses are, etc.
I always recommend a multi-disciplinary approach, though. Not just a pediatrician's ten minute check. In your friend's case, her child is kindergarten age, so she should be able to get an evaluation through the school system, even if she's in a private school, or not in school at all. They operate on a triage system, though, so evaluation might not be immediate.
Private evaluation is not cheap, but I think the information is excellent. In my case, I have a 23 page report from my son's evaluation, including recommended modifications and accommodations. I consider it well worth the money we paid.
But be sensitive to your friend's cues as well. Learning your child has something 'up' is like a mini-death, and the parents go through all the stages of grief, including anger. And the research shows that most parents need to hear it from three sources before they 'buy' it. If you're the first, . . .
I would imagine the school will be on top of it pretty quickly too. Good Luck!