My mum's love affair

I discovered my mum is having a raunchy love affair with a man I knew well. The lover is a gardener in my school. He fell in love with my mum when she visited me in school one day.

My dad is a hardworking man but he has alcohol addiction. He would drink merrily and doze off like a baby. My mum's lover became close friends with dad. He would buy drinks and intoxicate my dad. Then he and mum would sneak into a room and make love. Lately he's became brazen.
He bathe together with mum while dad is in hall. While dad is sleeping he would smooch and grope mum in the kitchen. I'm in a dillema .My mum seem to enjoy her lover's attention. So should I inform my dad about this affair?