Never tell the abuser to stop because the abuse will continue.
Report abuse wherever possible.
If you suspect someone is being abused, speak to this person gently and offer help, such as creating safety/escape plan. If the victim becomes nervous, remind them they can rely on your support.
If you think it is impossible to stop abuse, you are wrong. There is always a way to stop abuse.
For example, even if your abuser does not allow you to leave the house, it is still possible to end the abuse. For example, if you occasionally order a takeaway, you could potentially leave a note asking for help when you order your food, without your partner seeing.