"...ingredients with the word “sodium” in them*" are out..."
Like the sodium in lye -- sodium hydroxide?
"...Her washing machine drains into field lines in her yard, not a septic tank or sewer...."
I have to say that's pretty unlikely. All properly designed home (private) sewage systems have two main components -- a septic tank and a drainage field. Sewage from the house drains into the tank first. The tank is there so solids can settle out and microbes can have a quiet warm place to chomp up all those wastes. Then the cleaner wastewater drains from the tank into the drain field and from there into the soil. If her sewer water is draining directly into a drainage field without a septic tank, she's 99% likely to be in violation of local environmental quality laws. Not to mention the drain field would plug up pretty fast without a tank to remove solids first.
I spent about 5 years working in an industrial wastewater treatment plant and have lived with a septic system for over 15 years. On that basis, while I agree that large amounts of some typical household chemicals (including soap) could be toxic to a septic system, the modest amounts used in normal bathing, clothes washing, and such are fine.
Some chemicals have no place in a septic system -- petroleum products, paint, flammable products, pesticides, more than trace amounts of grease and fat, and large amounts of food from a garbage disposal. It's also good to minimize wild fluctuations in how a septic system is used. For example, draining a hot tub into a septic system can disrupt normal septic and drain field function simply from the unusually large flow of water.
In Alabama and the rest of the eastern 2/3rds of the United States, there is enough yearly precipitation that sodium and boron (the chemical of concern in borax) will not accumulate in the soil. In these regions, the rainfall exceeds the rate of evaporation and plant use of water. In arid or desert regions of the US, the story is different -- sodium, boron, and other chemicals toxic to plant growth and to the soil can accumulate over time.
To give you an example based on living in Iowa -- we keep a salt block out in the horse pasture for the horses and wild deer. It's pretty easy to see the effects of sodium overload on the soil around the salt block -- nothing, not even the toughest weed, will grow in a 3 foot circle around the salt block. It takes 2-3 years for the effects of the sodium to disappear once the salt block is gone. In almost 20 years of living with a septic tank and in the three years I've been making lye soap and using laundry soap, the soil in my drain field is NOT in any trouble -- the grass is growing quite fine over the drain field.
http://www.epa.gov/septic/brochures-and-fact-sheets-about-septic-systems-homeowners
I have been wanting to make foam soap.
