You might suggest that if she uses regular laundry detergent (e.g., Tide or Whisk or other major brands) for dog bedding that she try washing the bedding in a free-and-clear (hypoallergenic), or all natural (7th Generation, Bio-Kleen, Ecover, etc.), or even baby, laundry detergent and not using fabric softener (use white vinegar in the rinse cycle in the softener dispenser, instead). The vinegar helps get all the soap out, leaves fabric soft, and the smell vanishes in the dryer. Many humans are allergic to the parfumes and optical brighteners in major brands of detergent, so dogs could be too. Dog park dirt, which you mentioned, is also something to be suspicious of, as a lot of cities dump loads of pesticides and fertilizers on their parks--but I'm guessing she got a bath to get the mud off right after you left the park.