The friend with two passports should just make sure to use the same one on returning to the US she did entering the UK, and not mix them up, but she probably knows that.

Not true. Most of us with two passports use one for one direction and the other for the return. They don't have to match at all. In fact, I'm supposed to "mix them up". I have to use my American passport going to the U.S. and my French one entering the EU (I don't necessarily enter through France since I'm in a border area). My French one expired and I was actually scolded for arriving in Germany with my American passport and my French identity card. He let me pass after a tongue lashing in German (wrong language but hey...)

If she's born in the U.S., she'll get away with just using her American passport. She does have to show the airline consistently the same passport, but if she forgets, it's no big deal. They'll just ask since they're used to people with two nationalities.

Good point about the security screening but the same logic applies. While going through security on the way back, if either is selected for additional screening, the airline staff will not let them just be lead away alone. Security will keep the agent with the UM's. It's a female security agent who would do it, not the airline personnel but the agent will not be far. It's highly unlikely to happen anyway.

U.K. Customs also will consider both girls to be visitors, regardless of nationality. It's by where they live. It doesn't make any difference unless they're bringing a LOT of expensive presents, which I don't recommend they do anyway for a number of reasons

I'm glad they're UM's because that means it's less likely anything can go wrong. Many of the UM's we had on int'l flights were divorce cases visiting the other parent. We often got attitude from them. While we sympathized with the fact they were not happy about making the trip, we did have to keep them in line. If the airline staff seem kind of strict to the girls, this is why. I've had UM's try to sneak off, etc. Tell the girls, you're going to have fun but the other UM's might be be in such a good mood. Don't joke around or give the staff attitude. Set good examples for the other UM's, who will probably be younger.