Florida has what is called the "Felony Murder Rule" which is outlined in Florida State Statute 782.04.
This is applied two ways, either the offender kills somebody in the commission of a dangerous crime (like armed robbery) (whether or not the intent was to kill somebody), or as in the case here, where the offender is a participant in a felony where death results, regardless of that person being present at the time of the crime.
Here Ryan Holle was prosecuted under this rule based on his knowledge of the robbery and providing an instrument in the robbery (the vehicle) and knowing that a robbery was going to take place. In Florida, this means that because he knew of the robbery, and provided the vehicle used to get to that robbery, that he was an accomplice to the crime and stood the same charge/conviction as the people actually present.
Holle's sentence was commuted to 25 years by Florida Governor Rick Scott. He was originally offered a plea bargain of just 10 years but refused, and was subsequently sentenced to life.
It would not have been the same if Holle had lent his roommate socks or a comb because they didn't use them to commit the crime. Florida does not impose upon its citizens the duty to report a possible crime, so just knowing about the crime is not enough. Holle aided in the crime by knowing what the car was going to be used for and letting them take it anyway.
Florida looks at it this way: Holle lending the car to the roommate to commit the crime is the same as if Holle had given his roommate a gun knowing that the gun was going to be used in a robbery. The robbery resulted in the death of an individual so the person providing the means to commit that crime is just as guilty as the person who committed it.