How to make selection from AppleScript dialog list with keystroke?
AppleScript is not robust enough to do exactly what you're asking, however if you want to be able to select by number then put the number ahead of the name of the fruit, although enter will still need to be pressed after the number is pressed. Otherwise, pressing whatever the first character of the list item entry is will select it too. 

For example, change the following two lines of code from:

set FruitList to {"Banana (#1)", "Kiwi (#2)", "Mango (#3)", "Coconut (#4)", "Pear (#5)"}
set FavoriteFruitAnswer to choose from list FruitList with title "Fruit Dialog" with prompt "What's your favorite fruit?" default items "Banana (#1)"


To:

set FruitList to {"1. Banana", "2. Kiwi", "3. Mango", "4. Coconut", "5. Pear"}
set FavoriteFruitAnswer to choose from list FruitList with title "Fruit Dialog" with prompt "What's your favorite fruit?" default items "1. Banana"




As a side note, double-clicking a list entry to select it with the mouse does also trigger the enter key press automatically.