Over the past year and a half DD formed a close friendship with a girl, who at the start of this year invited her, together with a couple of other friends on a holiday abroad. Plane tickets were subsequently bought. Over the course of the last couple of months however said friend began to be increasingly unavailable spending more and more time with another group which upset my DD as they had been so close. However, they were still on friendly terms. A few weeks ago my DD received a text from the friend to say she was really sorry, but because she would still be only 17 (she has an August birthday and the holiday is in July), she wouldn't be able to come to the resort, as the regulations were such that anyone under 18 had to be supervised by someone over 21. DD was naturally upset, didn't really believe the excuse, but took it on the chin and asked her if she could please let her know if she invited someone else so she could try to sell them her plane ticket, which the girl said she would do. Yesterday she found out that someone else had indeed been invited and they had already bought their ticket. Ultimately I am sure my DD will just have to chalk this up to experience, but part of me wants to take some action, like contact the girl's mother (whom I've never met) to let her know how disappointed my DD is and that she is now substantially out of pocket - not in the expectation that this will achieve much, but doing nothing about it feels like said friend has got away with treating my DD badly without consequence. I am sure there must be other people who have had a similar experience. What did/would you do?