Can the last two player heal with beer when Ghost Town is in play?
The question revolved around the following limitation:


  Beer has no effect if there are only 2 players left in the game; in other words, if you play a Beer you do not gain any life point.


(Emphasis in original.)

As such, it comes down to the number of players left in the game. According to Ghost Town, and reinforced by this FAQ, each ghost is in play until the end of their turn, to the point that they can even be considered winners and can cause the Renegade to lose.

As such,


On the Sheriff's turn, there are two players left in the game. The Sheriff can't use a Beer.
On the "live" Outlaw's turn, there are two players left in the game. The Sheriff can't use a Beer.
On a ghost's turn, there are three players left in the game. The Sheriff can use a Beer on the ghost's turn (in reaction to a lethal hit).