Biting toddler....

No it's not Mackenzie thank goodness!!! We have friends who have a little boy who is just two weeks older than Mackenzie and they seem to think it is okay that he bites. I watched in horror at dinner tonight as his parents allowed him to chew on their arms/hands as well as his older brother. Not once did they even bat an eye, in fact one time the dad held up this biting toddler while he was attached to his arm!!! I was horified...I was afraid to let Mackenzie down to run around. I don't want her to get bitten and then learn to bite. I would be mortified if she bit someone. That is a big no no in our book! So, here is my dilema...do I just keep Mackenzie away from him...do I forbid them from brining the child to my house(I had to do this with their older child last year)...do I voice my concern to her...what do I do. They are good friends, but their children just run wild and I will not have my child pick up those habits. I have a hard time because she is a good friend, but I guess I must think of my kiddo first. I can't stand ill mannered children.