Well, we don't know the details, we don't know what happened and society tends to put love over friendship, but that still sucks ass in big time. Her enmity is largely understandable and justified, she could be a bit more forgiving, since this is a teenage girl in that case, and at the very least shouldn't put salt on these wounds, but emotions and stuff.
I mean it's not really justified at all her friend was a grown ass woman who entered into a relationship with a child any consequences that spring from that decision are wholly her own responsibility. Holding Mishio responsible for how things turned out is ridiculously unreasonable and petty.
Unreasonable, yes. Petty, well... hmm.. When your OP put it in the terms they put it in, that kind of makes sense. If someone I knew for years, whom I loved and shared time and life with (platonic or otherwise), off and killed themselves after meeting their lover, I'd be pretty miffed. A reasonable adult would try and talk with their lover maybe and work out the details...maybe. However, grief is a massive monster that can change people for better or worse. It can cause people to lash out, look at friends and loved ones differently. Things tend to fester and fast. Thoughts and assumptions can race to finish without ever looking at the context.
All I'm saying is, while this is a plot device, that spite and malice is coming from a place of hurt and grief. It's very easy to call it petty or unreasonable until you see it for what it is. Hard to be mature sometimes when you're hurt.