It's a void function. You don't need to use return at all. In your first example return used to end function prematurely. Your second example actually will never execute cout<<reverse_str[i]; routine, so it won't work at all.
The difference between my code and your code is that I have one return statement while you have two statements: the first one is the return statement and the second one is the expression statement. The expression statement will not be executed because the control exits the function.