You may be able to minimize damage, though, in two ways:
1) Have a talk with your friend. Say that swearing isn't allowed at your house and see if she can talk to her kids about that. Depending on the ages of her children, they may understand and tone it down in front of your kids. Then again, they may not.
2) Have a talk with your children about proper language use and what's allowed in your family. Tell them that they may hear other kids say words that aren't allowed, but it doesn't mean they can say them. Again, this may have mixed results!
This is kind of what I was getting at, even though I never really said it.