Oh my son did this too! He did this for over a month while we tried different ways to stop this, he painted himself, his bedroom furniture and the walls. The ONLY thing that got him to stop was a Cool shower to clean him off. The water was not ice cold, but it was not warm either - just enough to be uncomfortable and make him not want this again. It took TWICE and the second we made a touch colder and it was DONE.
As for the advice - it's tough giving it if it is not solicited. With what I reccomended above, that's easy! "Hey sis, I read a story online about a woman who's son played with his poop for a month or so and she used a cool/cold shower to stop him -she said TWICE and it was done and over." She will say something like "Wow, really? I never would have thought - hmmm it seems a bit harsh." You follow up with "Yeah, but it is SOOOO unsanitary for them to be doing this - I think it would be worth a try if I were in that situation." Or something to that effect if you are willing to suggest this to her. That can go with out you telling her what to do, however in general, esp with potty training the unsolicited advice can be stressful.