Appaloosas carry the genes Lp and PATN which control the spots and characteristics of the Appaloose breed. Most Apps will change as they age. So, yes, it is similar to going grey or roaning out. My grandfather raised and bred Apps for years and had a foal born solid bay with a single speck on her stomach. By the time she was 5, she was solid white with varnishing (varnish= Appy roaning) on her frontal bones and legs.
Your friends mare will most likely be lighter every time you see her.