Step 1 - Have a heart
You can be a big help to your friend just by listening to what they want to say, showing that you believe them and showing that you care. Some of the things they say may shock you, especially if your friend seems tough or strong, or if you know the person who has been toxic or abusive. But remember that anybody can be abused or be abusive, no matter how they seem on the outside.
Why is this helpful? You might be the first person your friend has told and they may be feeling low, sad or angry because they are being bullied, hurt or controlled. By listening, you are giving your friend a safe space to speak and your caring response will help them to cope.