Yes McDonald's does fry their McDonald's Fries in beef fat.
The McDonald's Fries are fried in a combination of cottonseed oil as well as beef tallow which is also known as beef fat.
The Beef Tallow or Beef Fat that McDonald's uses for frying their Fries is a rendered form of beef or mutton fat, and is primarily made up of triglycerides.
The beef tallow is solid at room temperature.
And unlike suet, beef tallow can be stored for extended periods without the need for refrigeration to prevent decomposition, provided it is kept in an airtight container to prevent oxidation.
The beef tallow is what helps make the McDonald's French Fries taste really good.