OK. I am now 28. When I was 18, I went out with a guy for 6 months who I fell for absolutely. I mean, I thought the sun shone outta his ass. He treated me like crap and dumped me after my exams.
I cried almost every night for 6 months. I was inconsolable. However, he left me alone and I didn't have to see him every day, and I got over it.
I would say, just be nice and friendly. BUT if her ex is continually calling her and screaming at her, this rings alarm bells for me. Was he like this before they broke up? If so, it could be that he has developed a "need" to be abusive towards her and the daily calls is a way of him getting his "hit" of abuse at her expense. However much she may still care about him, she needs to cut him off change her number and get a restraining order against this guy.
It is hard, sometimes when you leave someone and you would rather not have, any contact with them is better than none at all, but this just makes things worse.
if she is talking about suicide then you need to take this seriously. Contrary to waht Code and others say, the old wives tale that anyone who talks about suicide by definition isn't serious about it is total bullshit. People who off themselves tend to obsess over the topic for months before hand. I think this is because we as humans have a natural instict to survive, and it takes a lot of mental conditioning over a long time to override that, no matter how shitty things may see.
What someone who is talking about feeling suicidal is unconsciously doing is saying "I feel I have no option but to take my own life. I want you or someone else to give me a reason to believe that I DESERVE to stay alive".
It's common for people who have been in relationships where the SO is controlling or abusive (eg harassing and screaming) to feel suicidal when the SOs approval is withdrawn, as they have become dependent on the SOs approval for self-validatino. They then feel they SHOULD kill themsevles as they think they are a loser and a burden. What you need to do is get this girl to improve her self esteem and see that she does deserve help, love and friendship, then she will understand that this guy is a loser.
Remember, a 17 year old's probelms are important to HER. And if she's not around due to suicide, she will never get to reflect on how unimportant these seem 10 years down the line.