I'm a 28 year old only child and my parents have been married for 30 years, both in their 60s.

They have had a satisfactory marriage but have always bickered on a daily basis. My dad has a short temper and verbally abuses my mum on a daily basis (as he did with me when I lived at home), which has over the years crippled her self-esteem that was already damaged due to her experience with severe mental illness in her earlier years (she's still on medication for bipolar today).

Today we all went out for lunch and when he was in the bathroom mum confided in me that he hit her last week, hard, in the forehead and that she felt "funny" for a few days after. When I was growing up dad would sometimes throw plates of food at her, or give her a small slap, but never a hard knock like she described today. She said it was the first time he's hit her and that it really hurt, she suspected she had mild concussion but was too embarrassed to go to hospital. It scared me to hear her say that because she's tough when it comes to pain, so when she says something hurts, it HURTS. She said she was going to call me but didn't want to "overreact" and that he offered to drive her to hospital straight away but she refused. I wanted to confront my dad but she insisted that I not say anything to him because she "didn't want an argument". I told her if it happens again to phone me straight away and I'll come pick her up and bring her to my house. I was alarmed that she left it until a week later to tell me what happened as we speak several times a day on the phone and are quite close.

She promised to tell me if it happens again but I fear that she'll keep it to herself after seeing how saddened I was to hear that she'd been hurt. They both see the same psychiatrist and I suggested that she, he or both of them tell their shrink about this, she shrugged this off and said she doesn't want to make a big deal of it. In her head it was an isolated incident that will not occur again yet it seems neither of them are taking the appropriate steps (eg: counseling, anger management) to ensure he keeps his temper under control.

Should I be worried that this will develop into a pattern of abuse? I've heard the saying "if he hits you once he'll hit you again" but is this always the case? He seems to be growing moodier with age but at the same time can see her point that it's been the only violent incident in 30 years of marriage. He also hit his mother when he was in his 20s which worries me too.

If anyone could give me some advice that would be much appreciated. Thanks!