Am i allowed to send a message anonymously on Facebook messenger?

Hi! So I need some advice on whether or not I am legally allowed (or by Facebook guidelines) to private message someone anonymously on messenger. I was recently dating a guy for quite a while and later found out he had a girlfriend. It s a difficult situation and after a very long time thinking about it and taking advice from others, i feel like the girlfriend deserves to know. I m not revengeful or spiteful, but feel morally it is the right thing to do. However, I don t know her personally and don t know how she will react so I would like to message her anonymously on messenger (so as not to implicate myself in any trouble). Is this allowed? If she did message back and want proof, am I allowed to send screenshot conversations as evidence? I don t know if this is a breach of Facebook standards or not. Thanks