TW: suicidal thoughts. I really need advice on this.

So my best friends sister, (who's also my best friend) told me that her sister is having suicidal thoughts. This came as a complete shock as I have never noticed anything that would make me suspicious of her mental health. I've been going trough our messages on tt and WhatsApp but I just don't get it. I can't give all the details bc I'm worried she'll find this somehow. Please help me. I had a suicidal friend in primary school and I fucked up by thinking he was joking when he told us that he tried to jump out of a window. I really don't know what to do right now and maybe I'll delete this later.

I promised the sister that I wouldn't say anything to her but i had class with her the next period and I felt like I wanted to cry (which I almost never do. Idk if that's healthy or not but idrc). I managed to get trough it though without mentioning anything.

I will concider replying if you guys need more info. Thanks for now.

(Sorry if some of my English isn't grammatically correct, it's not my first language)