my dad is cheating on my mum what do i do?

me and my dad went on a week trip to japan, and were 4 days in. around 20 minutes ago he went out to smoke and gave his phone to me. i was mindlessly going through it when i saw messages of him texting someone, a random woman with a bunch of hearts and i loves yous, even pictures of food we just ate. im still in complete shock. ive known him for my entire life. we still have to spend 3 days in japan together before we go back. what the fuck do i do?