If your friend is American with no experience of British education at all perhaps you should provide a glossary of British terminology.
We tend to use the same words as the US to describe different things. For example the terms "grammar school", "high school", "public school" and "prep school" have completely different meanings over there. I can see the potential for major confusion!
Also, at least in the state where I lived, the way places were allocated (and funded) within the state system was completely different to in the UK. If the family are considering state education (even as a fall-back) it is important that they know that they are NOT guaranteed a place at their local school. Also, there will be no help with the costs of books, transport etc if they choose to go private. I'm assuming, however, that if they are looking at paying potentially £50,000 - £80,000 p.a. in school fees the latter is not an issue...
For what it's worth, I don't think the scramble for private school places is as bad in London as in NYC. And I'm guessing other big US cities are pretty bad too, so it won't necessarily come as a great shock to them.
Oh, and make sure they know (though it sure to be in prospectuses etc) that the relevant date for deciding which year group a child goes into is 1st September; I think it's different in the US. Although I guess these things might vary from state to state so I may be making misassumptions.