You're not an asshole at all. As a matter of fact, you've done almost everything right. Even the public post was understandable, even if it probably wasn't the best thing.
I don't have any advice for you. I've been through this, it happened to me. Except when your Laura messaged you back, mine didn't. My best friend for years just disappeared one day and told me to stop contacting him. I sent him messages for a month and he never replied. He had recently started seeing someone and she wasn't a big fan of me, on account of our history. She had never even met or spoken to me, but it didn't matter. His brother and mine were friends and held their graduation party together and he didn't even come. His mom introduced me to her friends as her "should-be daughter in law" (even though I was dating someone else by then). It was a mess.
He married her within a year. My family was banned from attending.
Six years later, I got a text message. I had never deleted his number and my heart stopped when I saw the name. He sent me a long apology. He seemed genuine, I eventually figured out that they had gotten a divorce. I don't know the circumstances. It was the closure I wanted, but to be honest, 6 years was too late. We're totally different people now.
I don't have any point in telling you this, just wanted you to know that someone else has been through it. In his case, his wife was very controlling and probably manipulative. While that is a form of abuse, I'm not sure that it ever really crossed into a dangerous territory (as far as I know, he was happy for most of it, no depression, no threats, etc). Your friend sounds like she's in a lot more trouble, but you've done what you can by telling her mom what you know. Express your concern, but do what's best for you.
Similarly, I was in a different situation where I threw my life away for a year trying to save an ex who was suicidal. I still to this day don't know if I did the right thing, most people say I didn't, that I should have protected myself and let him go, but I couldn't have lived with myself if something happened. It cost me a lot, and going down that path is the worst thing I've ever done. I can't recommend sacrificing yourself like that, but if you choose to pursue Laura and to support her and burden yourself with her depression, just know that someone has been there too.
That was all ten years ago for me, and it took a long time to get past all the baggage that those people left with me, but I did move past it. I hope you can too.
P.S. I have a wonderful drama free relationship now and fully intend on living my life happily ever after, despite the scars. No matter what happens, time will make things better.