5 year old potty mouth?

My husband is the custodial parent of his 5 yr old boy and 7 yr old girl. His son knows what a dirty word is and he knows he's not permitted to say them. When asked where he picks up these words, he says that Mama swears all the time. When we confronted her about it, she said that "she'll speak as she pleases..the kids know that they aren't allowed to use those words". When we catch him swearing, his father punishes him by and soap in the mouth. The whole time this child will tell you he hasn't done anything wrong, even if you heard the words with your own ears! What can we do? He doesn't use like your standard dirty words like poop or fart, he uses words like c**k, sh*t, f**k, etc....WE do not talk that way or use those words in our home!!