my dad is sleeping with a girl the same age as I am

My dad started bringing this homeless chick & her boyfriend around & told all of us that they were in their 20's but the girl is actually the same age as I am (18) & it makes me extremely uncomfortable. No one knew they were together on the low since she's in a relationship and then my dad started bragging about it to everyone in the house. My dads friend took his phone & texted her boyfriend and began to act like it was my dad texting him & told him that he was fucking his girlfriend & after he found out he got mad & then the girl told my dad that she was calling the police on him for rape and what not, he started stressing the fuck out over it & cut them both off then the girls boyfriend gets locked up & now she's back here a week later.