Additionally, Dani says 'let me know' as if they're half-made plans.
I've literally had the same thing just happen - a friend messaged me last night to ask if I wanted to grab a coffee today, I said sure and where/what time. She had something else to do first, so I said to just let me know when she's ready and then I didn't hear from her.
We haven't met up today as I hadn't heard from her (but I have made contact this evening!!), but it wasn't a huge concern to me because the plans weren't set in stone and... It just wasn't a concern to me!
Hindsight is an amazing thing. I think too much inference is being put on why the friend didn't raise the alarm sooner. For one, I bet she wishes she had. For two, I think this only becomes such a concern in retrospect. And for three, there's absolutely no suggestion from anyone that the friend is involved in some way so I'm not sure why this keeps being brought up as if it's key evidence...? I almost feel like it's pretty unfair on someone who probably feels just about as awful as they can feel in the first place.