Mac already had MH and drug issues didn’t he, before Ariana was even in the picture?
I’m NGL these in general aren’t Ariana’s responsibility, but if she was cheating on him then you can say she is responsible for hurt that exacerbated his mental illness, but it’d be too far to say she literally killed him.
Equally though, if they broke up and she didn’t do anything wrong, you can say the break-up had an effect on Mac’s MH but at the same time its not really fair to look badly on Ariana for breaking up with him.
Just putting myself in Lilly’s shoes, I don’t know how well I’d cope if I saw Ariana’s PR and my ex husband trying to make it sound like no cheating was involved. It’d be like being gaslit on a massive scale. That’s what’s the most scummy of everything like they duck up her life and then try to lie about it to make themselves look innocent, from an ivory tower? It’d make it so much harder to move on