my bestfriends wife

i truly have respect for my bestfriend, he's open minded and he's like my brother. But her wife is my biggest fantasy. her name is Cassandra. i think she's so hot and she makes me hard as fuck. my bf told me that she's a real slut with him and it makes me kinda jealous. i'm very closed to them and i feel like i could fuck Cassandra easily. what should i do? all i think about is her pussy