We tried straw bedding deep littered as a cost-saving exercise about a year ago. It was awful. The beds were a nightmare to keep clean, they always stank, and we had to take out so much muck and put back in so much clean bedding. We went through 2 big bales then went back to wood pellets and everyone was happier (except for the pony who though having a bed he could eat was marvellous!) A straw bed can be cheap and looks lovely well done, but needs constant attention.
I'm going to play devils advocate here and ask if they definitely hadn't been cleaned for 2 days? Because even one that was mucked out 8 hours previously can look and smell awful on straw. But if she isn't mucking out between you coming, that's out of order. It's bad for the horses especially if they don't get any turnout, and it's bad for you as with 3 horses that's effectively 6-9 days worth of shit you have to move.
I'm also going to go slightly against the grain and say that if she is struggling cash-wise, rather than ditching decent feed, back people, etc, she should remove a horse from her little herd. Even if it is just to loan it out temporarily. Please tell me you are not paying this lady for the privilege of mucking out her horses 3 times a week? If you are you can expect her to pull her weight on the days you don't do.
Finally, a bit of a thread derail - "most amateur-owned horses shouldn't need feed with sufficient, quality forage." I'll tell my 3 (even the pony) when they've done a 4 hour day's hunting that and see how they like their hay 
. My horse in particular likes his oats and sugar beet. He competes over long distances in the summer too, and is by no means unusual in doing that (I don't know many professional Trec or long distance riders, they definitely fall into the amateur camp.) Even my daughter's Welsh C needs a bit of feed to keep him pinging round Pony Club rallies and camps.