Help me plz

Crosspost "My [23F] boyfriend [21M] is hiding me from his ex and the reason for doing so is shocking" from /r/relationships:

We have been together for slightly more than a year and have been in a half long-distance relationship (half of the year he is at home (Australia), half - abroad studying (the UK)). Overall our relationship is great: he takes care of me, is a nice person and I am in very good terms with his family. The only thing that was bothering me was his strange behavior on social media.

Before jumping in, I have to say, that he isn't a very active social media post-er, but he uses both Facebook and Instagram everyday, mainly for keeping in touch with family/friends and checking out memes/photos. I, on the other hand, am quite active: I post photos on Instagram of my travels and a bit of my daily life, use Facebook and other social media. I am active, but not in an obsessed kind of way.

So. Everything started off by him untagging himself from our mutual photos and deleting my comments under his photos. His explanation for doing so was: "I don't want my exes stalking you". I was quite bothered with this statement, so I created a reddit post and got very decent advice from people. One of them being something along the lines of: "He gave you a shitty excuse. Talk to him and find out the real reason behind his behavior"... So I did.

What I found out next was pretty shocking.

Turns out, before meeting me, he was in some sort of online relationship (not sure if they were official or exclusive) with a girl from Canada. They met online and never had a chance to meet, until she decided to move to where he was studying and attend the same university as he was. Basically, they were planning a happy future together, that was supposed to have begun in September 2017, but then he met me, when he was back home for the summer - in June 2017...

From what he told me next - he told her, that he wasn't interested in a relationship. Apparently, she even knew about my existence and when their communication slowly began to fade and she got curious, why he was always talking to someone, when she was calling, he told her that he was just talking to "a friend" - me. He told her not to worry and that it's purely platonic. Yeah. Platonic.

Then he proceeded to "cut her out of his life", by ignoring her messages and multiple calls. When he finally picked up the phone, he told me that she was sobbing and yelling at him, demanding an answer. He found it to be "enough", so he blocked her everywhere.

Even though he didn't say it upfront, for me it is clear, that he gaslighted this poor girl, lied to her and dumped her in a very cruel way.

In my eyes, the reason behind his strange social media behavior is her. He doesn't want her to know that I am his girlfriend or he is trying to make me think, that she is the crazy one. Even though, she clearly isn't.

I really hate being hidden from the rest of the world, as well as the fact, that this girl deserves to know the truth. Her behavior (she texts him occasionally, calls him, stalks me on social media) suggests, that she is oblivious of what happened and probably still thinks of me as "just his friend".

Dear Reddit, I have a few questions: what do I do, do I tell her the truth and what is that chance, that they are still talking and this is just a double play (something that has been suggested by some of my friends)?

TL;DR My [23F] boyfriend [21M] is hiding me from his ex and the reason for doing so is shocking - he gaslighted and lied to her about not wanting a relationship, then started dating me.