I've heard rumours, and they're just rumours, that they will hunt you down (so to speak) and make you pay the difference between the return and one-way costs. I can't see this being realistic.
Before I posted my reply, I did have a quick look online to see if there were any problems with missing the return leg, and I also came across something about the airline being able to charge the difference, but I don't know that it's actually something that happens in reality. Plus, they would only be able to do that if they could track her down.
When I was living in the US, I bought a return ticket to move back to the UK, and just didn't use the return portion back to the US again. That was in 2004 and nothing ever happened because of it. Initially I had booked a return going from the UK->US->UK but was only planning to use the second US->UK part, but it was when my mum talked to the airline that they told her I couldn't do that because if I missed the UK->US part, they would cancel the second leg. So, I got a refund on the ticket and purchased it going the other way US->UK->US and just didn't use the second part.
And yes we have thought about this. However, every trip out of the UK will always be a return now, so taking the return leg will put things out of sync.
Yes, that is a good point and something you would have to consider - plus, it's often more expensive to fly from the US to the UK than the other way around (strangely).
I think we'll just take the risk. Is it worth informing them you won't be able to make the return, or just not turn up? I'm thinking the latter
I wouldn't say anything to anyone, just don't turn up for the return leg. If they know in advance that she won't be travelling back to the US, they may try to charge her for a one-way or something similar.