cheating dad...😭

My dad is cheating on my mum and I'm the only one who knows...my dad has a tinder profile and many other dating accounts even though him and my mum have been married for 13 years and He is asking women for nudes and calling them sexy and cute and I'm not sure if I want to tell my mum cause she has tried suicide before and I don't want to break up there marriage and is it even really cheating?!?!