How do I get my married friends to fight fair?

One of my best friends and her family has come to stay with us for a while. I the problem is is that they fight really mean. Yelling at each other to shut the **** up and calling each other names.They don't seem to care even if I am in the room. I suggested counciling so that they can learn how to talk to one another lovingly instead of behaving like complete enemies. My husband and I are lucky to have matured in our relationship and we discuss our problems rationally. The other part of the problem is that their four year old is wild, is disobidiant and mouthy. I know that my friend grew up in a rough situation and didn't have role models she could count on. She cusses at her child and calls him names sometimes, threatens punishment and never fallows through, hands all of the discipline off on dad then seems frustrated by the lack of respect. I can't help but thinking she is really hurting him even if unintintionaly. All of this insantity could be avoided by simple kindness.