Social media often seems to blur what would usually be considered very normal relationship boundaries.
‘Meeting’ someone online isn't the same as a friendship developing naturally over time through work or a hobby.
It begins as one-sided interest, with one person unaware that the other even exists...until they choose to ‘introduce themselves’ by reaching out to the stranger on the screen.
Even if they communicate publicly on social media, this is still a superficial connection. The only way it can develop into one-on-one, deep, personal conversations if one person chooses to go to DMs in order to start a private conversation, and the other person reacts favourably to that message.
I’m sure some people would disagree, but as far as I’m concerned, noticing a woman online, having a public interaction, then moving to private messages, is just as much of a betrayal as going on a night out, seeing a woman he finds interesting across the room, approaching and chatting to her…then asking for her number and messaging her the next day (or her approaching and him giving his number to her).
It’s dodgy as fuck, and very obviously crosses a line.
'Wanting to offer support to a stranger' wouldn't be a valid excuse for doing this in real life, and I don’t believe anybody who tries to claim naivety, thinking they were “only being friendly”, just because they approached/were approached over the internet rather than in a bar.
Actively pursuing a friendship like this, especially as he’s continued messaging her for 18 months without ever mentioning his conversations with his new friend, or asking your thoughts while he supports her through something you’ve experienced (apart from, apparently, once over a year ago) suggests he knew you wouldn’t be happy about it. It doesn’t suggest he thought this was a normal, totally above-board friendship that you’d be fine with.
She might believe it's totally innocent, and there's every chance she'd be mortified to find out you'd never even heard of her. If she thought it was a genuine platonic friendship, there's also no way she wouldn't question his motives after realising he's kept her a secret from you. I hope this is the case.
This isn’t a normal friendship, and being upset is a completely valid reaction. You have nothing to be embarrassed about, and you’re not unreasonable for finding it unacceptable. He’s the one who should be embarrassed.
Hope you’re okay OP.