Think I would check for mold behind the walls in the bathrooms. A musty smell could be coming from mold somewhere.

Friends of mine have that front load washer/dryer set and they went through this same scenerio and it turned out to be the washer. Apparently it will get musty because the water sets in the bottom -there are a few products out there that are made just for this problem. What ever the cost is it would be more then worth the price. Also dishwashers can do the same thing. And they also have a product for this too.

Our front load washer used to have a musty odor when we used liquid laundry detergent. The products sold to get rid of the odor, didn't work. Had to switch to powder detergent to get rid of the musty smell.

If she has a high efficiency front load washer, it may be from that....

Wow this really sounds like a drag. Is she able to pin point where the smell is coming from? You have checked all the water traps to make sure one has not dried out? The drying out of traps is more a sewer smell then a musty smell. My nose tells me you have water pooling somewhere in the house. Since your friend lives in Phoenix Az she probably does not nave a basement. It sounds like the professionals need to be brought in. You have had plenty of suggestions here. Please let us know what happens. I live in south Phoenix and I'm curious.

While she is waiting to find the source of the problem, a citrus oil air freshener can help with the odor situation. She might also try using a citrus based cleaner when the source is located.