I believe that some people are so set on committing suicide, that whatever you do or say will not deter them. Not all suicides are selfish acts though IMO, as some people committing them are not capable of thinking logically to realise the heartache and pain they cause to people close to them when they do it.
I think the most tragic case of suicide I've come across is a bloke who used to work with Andy.
He had 3 kids aged between 5 and 12, and his wife had committed suicide 3 years earlier by hanging herself off the balcony on the stairs. This cut the bloke up so much he had loads of depression problems after that. To everyone he appeared as if he was finally getting better, he had a good job, a nice girlfriend and had just bought a new car after getting his licence back after a ban.
One morning he'd got his kids off to school and hanged himself exactly the same way his wife had. Little did he realise that his little lad aged 7 yrs had forgotten his footie kit and had come back home to get it and it was the lad who found his dad hanging

I can't imagine what affect this will have on the kids during their lives, especially as they weren't able to stay together and homes were found for the 2 girls with different family members and his girlfriend took on the lad. Having both parents committing suicide must be absolutely awful
There's no use in beating yourself up thinking 'what if' as more often than not, the person is so determined to end their lives no matter what.
Mark, I hope your girlfriend can realise that someone so determined to end it all, can't really be helped and she gets over this soon.