In the US we use the slang term "to ride the gravy train" to describe a situation in which someone is benefitting from a situation without having to pay for the benefits.
"He's riding that gravy train" means he is taking advantage of a good situation at little or no cost to himself.
A simple example: a person lives in another's house and enjoys a bed, the food, the TV, the computer, the beer
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...all the amenities...for free...at the generosity of the owner.
We would say to him, "Man, you should ride that gravy train as long as possible."
It short, it is a free ride. Is there an equivalent expression in Spanish?