I caught my Dad in bed with my Mum's friend

Yin replies

Although it not necessarily your place to come between your mum and your dad, as he should really tell, if he has shown no sign of telling her then I would let your mum know. Perhaps approach your father first or her friend, tell them what you saw, if they don’t know already, and give them the chance to come clean, if they refuse then it’s down to you. If this was happening behind your back would you want someone to tell you or find out later down the line?

Yang replies

It is going to devastate your mum, she may not believe you at first, because she may be in so much shock, but if you give her the details of what you saw then she will realise that it has happened. Take her somewhere neutral, like a café, because if she is at home with your father she may react aggressively towards him. If you are away from the house then she has time to absorb the information and calm down a little before she approaches your dad or her friend.

Reassure her that you are there for her, as the situation could get really messy between them; tell her that you will support her through it, because having both you and her husband taken from her would be a big blow.