Fat free evaporated milk is usually subbed for cream. I am pretty sure it would work, but it may depend on what she is making.
For example, if what she is making simply calls for milk (like a cake batter), skim milk or 1% may be a better sub.
Evaporated milk has a thicker texture, since much of the water has been evaporated out of it.
If it's calories, rather than strictly fat content, here's a milk chart. Tell her to avoid the reindeer milk!
If it's saturated fat, then it makes more of a difference.