If she's just cleaning the tank 'when it gets dirty' and parameters are an unknown, I think it's a relatively safe bet that these fish are dying due to ammonia/nitrite poisoning.
I would suggest a 50% PWC
*now* (be sure to match temp and use a dechlorinating product) and get a test kit (such as API Master Freshwater Test Kit- it's worth the $) ASAP and test for ammonia, nitrite, and nitrate. Do not get test strips- they are generally inaccurate.
Acceptable levels of ammonia are 0-.25ppm; nitrite, 0-.25ppm; nitrate, below 40ppm. Ideal levels of ammonia and nitrite are 0 and a healthy, mature tank will have nitrate levels of above 10ppm (regular partial water changes are needed to keep nitrates under 40ppm). If ammonia or nitrite is over .5ppm, a 50% water change is in order. Your friend will need to test for ammonia and nitrite daily until they have remained stable at 0ppm for several days, and test for nitrate regularly to make sure it has not risen above 40ppm (another alternative is to do weekly PWCs of about 10-25%).
Moving the gravel about isn't a bad thing per se; however in an unhealthy tank you can release enough waste to cause an ammonia spike and kill fish...