Rather than adding a house rule for crossbows firing from prone, you could also just keep it as is and make the shooter get up on a knee ("stand up"), take the shot, and go prone again for cover. It makes it more of a gunfighter duel where people are shooting from cover. Someone has to pop up to take a shot, and someone on the other end can hold their action to take a shot when they stand.
Or someone can drop any one of a number of area of effect attacks to flush the sniper.
Or someone can drop any one of a number of area of effect attacks to flush the sniper.
