My dad is online cheating on my mum. My mum was the one to find out. She said I can’t tell anyone else but wants to talk about it and asking what she should do. What do I say?

Sit down with you mother and explain to her that you are there for her but as her child, you cannot offer any advice on her marriage with your father and cannot really take sides in her marriage. As a child of BOTH your parents, its not really your place to offer opinions and judgement on her marriage even though you support her and love her no matter what decision she and your father make regarding their relationship going forward whether it is going to therapy/counseling or reaching out to someone else. You can also tell her that you honestly don't know how to feel in this situation and given your connection to both of them, you don't think that you can give an objective answer.

I would politely explain that you are the child in this situation and you should not be the one that she discusses things like that with and that it is a private matter that she and your dad should work out with a professional. No parent should ever being such matters to their child regardless of their age or maturity level. Matters like that need to be handled privately with someone that is trained to help in such matters such as a couples counselor not a child and you do not deserve to be out in that situation to begin with.