Do they have a crawl space under the house? Could be something like pack rats under the house.

this sounds crazy but I know of a lady that couldn't find the source of where her problem was it just smelled musty she poured bleach down every drain every day for about a week and the smell left now she flushes bleach about once a week to prevent it happening again. she could try this untill she can get it fixed if it is a leak

If it is mold that is undetected, place bicarbonate of soda in containers in the suspect places. Half a teaspoon of oil of clove in a litre of water will kill mold spoors. Bleach does not kill them - they always come back if you use bleach. Spray onto the mould and wipe off after a few minutes. It will have a strange smell as well but it soon goes away. All the best with your problem.

If it is the drains, half a cup of bicarb soda and half a cup of salt. Put into the drain and add one cup of vinegar. Be careful as it will bubble up. After a few minutes, flush the sinks.,

Just a note, if you have a septic tank please don't do this, it kills the bacteria that breaks down what comes into it.

Since it is Phoenix in an ultra dry climate, has she had the swamp cooler/air conditioner serviced?
Otherwise I agree with the others water is leaking somewhere and needs to be stopped or dry rot will set in and it is like cancer, just keeps growing if the wood isn't replaced.

Here is another suggestion....could it be a dead rodent in the walls. Here we have lots of mice and rats who live out in the wild and would like to live in the house. I live in an old house an there is spaces between the inner and outer walls. I put out poison in places I know where they can come in. It is the kind that is supposed to make the rodents seek water and hopefully go out of doors to die. Well every now and then one goes into the wall space and dies there. It makes a musty smell in the house. I don't have a solution except to put out some cedar cuttings to freshen the air and wait a week or too until the smell leaves. It does make my son's Jack Russel dog go nuts though as he searches the perimeter of the rooms looking for the rat.

I'm thinking a mold inspection from a reputable company would be a great idea.

The air ducts and Central Air/Heat drip pans may need a professional cleaning.

Check for BLACK MOLD hidden where you cannot see it! I still suffer from conquesences from this. It was found around our air conditioner outside my bedroom door. When our AC was replaced the man took short cuts and did not install the inside unit correctly.

Best of luck, delma

I am sure this is a serious problem for your friend and I hope she gets to the bottom of it. But her situation did remind me of a humorous story involving a divorcing couple, curtain rods and revenge involving awful odor in the house. I looked it up

http://jokemail.blogspot.com/2007/09/curtain-rods.html

If she needs some light relief, please share. I hope she can find a solution that fixes her problem without huge effort and expense.