I've never had a problem, then again it might depend on what she uses to dye it or keep the color. Conditioners and such. When I do dye my hair it's usually with a product called overtone, so it's easier to keep the color. It's a colored conditioner essentially.
It depends on the dye used, wether it's permanent, semi permanent, or demi permanent, and what the slime is made out of. I've regularly dyed my hair different colors with semi permanent dye and the only time I've ever had an issue is with huge amounts of shaving cream that was on my hair for a long period of time. Never had any issues with any type of food product that I've used. If you're using something thick and sticky that requires a few shampoos to get out, the multiple washing's could lighten the color if it's semi or demi permanent. Hope that helps
I usually wait a week after I get my hair done but I get blonde balayage and have never had a problem. Sometimes I'll have to retone it but I'd have a to any ways. Its more that some substances have fried and damaged my hair than messed with the color.