I agree with Knibitz--hair conditioner and cold water.
Most human hair styling products leave a lot of residue--you don't want that on your doll. Some may react with the plastics or other materials that your doll is made of, potentially causing discoloration. Conditioner, particularly watered down, is often very effective at "de-frizzing" plastic doll hair.
Otherwise, hair products made for wigs are a good bet.
I've used some of these products, which are made specifically for doll wigs, on some of my doll's wigs--but I don't have any Monster High dolls. Volks
Water Wax,
Wig Mist, and
Oil Spray. These products are intended for wigs that can be removed from dolls, so if you do go this route you may want to cover your doll with a towel and apply the products to your fingers and then to her hair. That will help protect her body until the product is dry, rather than spraying all over her.
You want to try using as little as possible of any product (even watered-down conditioner), because if you build up too much and need to wash it out it may ruin the style you were trying to preserve.