Answer
Your friend is adding $3+2$, before multiplying $2(9)$. You should help him by explaining that in the order of operations, multiplication and division come before addition and subtraction. Use the acronym PEMDAS, Parenthesis, Exponents (and Square Roots), Multiplication/Division, and then Addition/Subtraction.
Work Step by Step
Your friend is solving the equation as if it were like the following:
$(3+2)(9)$
$(5)(9)$
$45$
He is solving it as it as if you solve from left to right. However, since there is multiplication, that is done before addition. Therefore, the problem should be solved as such:
$3+2(9)$
$3+18$
$21$
