What would you do if someone you loved was slitting their wrists would you support them and hope that they'll stop or would you tell someone even if it means that, that person would never speak to you again.

Did you mean self-harm, or suicide? If it's the latter...
That seems too hard and stark a choice for someone you love: Death or estrangement.
If it were someone I loved, let’s call him or her Beloved, I’d have to be damned sure that those were the only alternatives. That would mean a lot of talk and urging Beloved to seek professional help. It would be safe to say Beloved was in a dark place and not seeing things too clearly. I’d be worried that, being so close to Beloved, I might also not be seeing things right. So someone who is objective, independent and counsels people on a regular basis is needed.
And if – hypothetically – those were still the only options?
(To lose my Beloved to death or estrangement)
I’d go for the loss that left Beloved with possibilities, with hope, and with some chance of a life. It may sound flippant, but I don’t see death doing that.
I’d want my Beloved to be well and have a happy, busy life, even if I were not part of it.
I’d like to look forward to meeting Beloved in the street. To smile and say, ‘Hi, you’re looking well…’ And end with, ‘Goodbye, I’m glad I met you.’
_________________
Give me golf clubs, fresh air and a beautiful partner, and you can keep the clubs and the fresh air.
(Jack Benny)

Finding them professional help should be a priority.

Although it may not feel like it to them, self-harm and suicide is often a form of attention seeking, so ignoring them would be unwise.

thank you its helped me to understand the situation a little bit better.