How do you get 5 years worth of fast food grease out of long hair?

I have a friend who is developmentally disabled and works in a fast food restaurant. I have been trying to help her, especially with grooming. I finally asked her to let her long hair down from the ponytail she routinely sports, and discovered that her hair is coated with a thick coat of grease, that makes it appear that her hair is wet, even the day after washing. I washed it for her last night, and cut out a lot of knots that I simply couldn't untangle. I couldn't check the results when dry, but I'm sure there's lots more grease in her hair. I am looking for a quick solution so we can take her to get her hair cut and styled. I'm trying help this young woman and help her become a lady, she's really smart and has a good heart. Hygiene is definitely not her forte .... Please help!