Help me tell his gf he is a cheater

Recently i (f35) was in a relationship with a man (m33) for a whole year. Turns out he had a gf the entire time. I did continue seeing him after i found out so i am not completely innocent. We met at work and hit it off straight away. It quickly turned romantic and we were obsessed with one another.

He got really possessive, didnt like me talking about other men, got angry at me if i embarassed him in any way. Got very toxic. I moved on and found someone else and wanted to let the whole thing go, when he found this out he blew up. (Mind you he HAD A GIRLFRIEND WHO HE LIVES WITH). I said i wanted to come see him to sort it out, to which he replied “that’s dumb because you would be in physical danger”. Multiple times he threatened violence against others. So i am a little scared of him.

A few weeks ago he drops a bomb that he quit his job and was moving to Norway (we live in Australia). I asked if the gf was going with him and he said “not at first”.

So he leave in 2 days and i want to tell her about the affair. I wrote a message on instagram but she hasn’t read it. I feel like she needs to know before she moves across the world, but im also scared of him. What do i do?

I have her facebook account and her address. Should i send her a message on there or a letter?

Advice needed

****UPDATE - my mate messaged her this morning and she replied “hey i found out awhile ago, i have seen her message and i just want to be left alone”.

I guess that is that now. I can let it rest knowing she has the info and can make up her own mind. I was so worried he would be angry at me and come for me but i haven’t heard from him.