When someone you know is suffering from mental health issues it can be difficult to know how to start the conversation. That's why calm (campaign against living miserably) invented the ALAN method to help you find ways of helping your mates through a tough time how to use the ALAN method:
Ask open questions like, "how are you feeling?" to get things going show your BFF that you care without judging them about what they're going through
Give your friend time to speak at their own pace. You don't need to fix everything for them, they might just need to get stuff off their chest.
Sit down with them and get them to make a plan to seek help if they need professional support. Show them where they can get it by connecting them to a mental health charity like calm or offering to accompany them to the GP.
It's important for your mates to know that you're there when they need them, arrange a time for you to hang out in person... And build a big network of family and friends that they can all reach out to when they're feeling low.
If you need support, speak to the Samaritans on 116 123 and if you think someone is at risk of suicide, stay with them and call the emergency services.