I know someone with deplorable living conditions. What should I do?

There is someone that I am forced to deal with that lives in utter filth. They have a cat that they allow to do his business anywhere he wants and it is not cleaned up! It has caused them to personally smell so bad that I cannot stand to be around them. This is someone that I must deal with and would love to just turn them into the department of family and children’s services in my area (there is a small child involved), but if I did, I would never be forgiven. It has gotten so bad that just picking them up in my car has made me physically ill! Does anyone have any ideas? I’m getting desperate.