I think you should be extremely careful and sensitive about whatever you choose to do, because absolutely none of this is his poor girlfriend's fault. She has done nothing wrong. So if you do decide to message her, be kind, tactful, sensitive about it.
And don't do it while they're on holiday. It can wait.
Your motive here sounds very much like revenge on someone who has been stringing you along (and in a very obvious way, from the sound of it) rather than any concern for his girlfriend, who is actually the one who has been cheated on.
You haven't been 'dumped' by this man because you were not a couple in the first place - you say yourself that your relationship ended previously because he wouldn't go public/official, and that he has just been asking for 'dates' and you slept with him a few weeks ago. I think you need to be clear in your head that you are the 'other woman' here, not her, so tread carefully.
You dated him last year, but describe yourself as 'in love' with him, which is all very intense. I'm really sorry that you've been shat on like this, but I also think that if you decide to tell his girlfriend your motive should be kindness to her, not making yourself feel better.