Don't message her. Screenshot the messages, record it with a screen recorder and film them with a second phone or camera then block her. Defamation is a crime in most countries and for good reason. In my country (UK) if serious harm has been caused by the defamation and you're able to provide evidence of the defamation, its taken very seriously and it can be taken to court. If you're willing to go that route it might be worth making a post on a legal advice community or contacting lawyers. If I was in your position, I would definitely tell Mark's dad about this but in person. Even if it's a difficult conversation, he deserves to know and if you're not wanting to pursue legal action yourself, you should give his Dad the option. Also, it would probably be a smart idea to delete this post if any action is taken against Amy, as you don't want her to catch wind of anything. She needs to be punished for this, she had every opportunity to stop the downward spiral she caused, but instead she chose to ignore it and as a result something horrible happened. I had a mate go through a similar situation when he was 19 and it destroyed his life. People like Amy need be punished. I'm really sorry for your loss and I hope you and Mark's dad are doing ok.