I would explain that we dont use words like that in our house - ever. Just b/c they are "okay" at dads, they are not here.
I would give a phone call to the ex to calmly let him know that I dont approve, but in the end, you can only control what goes on in your home. Explain why using the words are wrong (insensitive, cowardly, swear words are for people who cant think of something intelligent to say, and ds, you're intelligent, etc)
We had an issue close to this with my ex. He would take ds out to the family farm and ex and his bro's would take ds out in the woods and teach him to shoot with a bb gun. Ds shot a bird. Blech.
I asked ex not to do it, he said he wouldnt stop, helping enviroment, blah blah bull. I cant stop him. I cant stop ds from seeing him. So, I talked to ds about what it meant to kill a bird - a living creature. I told ds that I would prefer he didnt play with the bb gun but since I'm not there, he has to make a choice. In the end, he still played with the bb gun, but he stopped shooting animals (just cans).
Kids will sadly pick up undesirable language any and everywhere, the best we can do is teach why its wrong. (which you're already doing...)
So, I say, sorta let it go, but treat it in your home. You cant change theirs.