If he’s putting other people within the shared accommodation at risk (through financial abuse), even without the risk to your mum, then you absolutely did the right thing.
With the additional risk to your mum, there is nothing else you could’ve done. You said your mum was terrified of being “found out” – found out? Was she trying to hide the extent of his addiction so people wouldn’t know and judge/shame her? Doing that will continue to fuel his addiction.
I’m sorry this is falling onto you, but please please keep in mind you are not responsible for causing this pain. The addiction is. I’m really sorry you’ve been made to feel so guilty when all you are trying to do is protect who you love. You are not in the wrong at all.