No, your friend is not correct. The remainder obtained from synthetic division when dividing f(x) by (x-a) does not give us any information about the value of f(15) equaling a.
Synthetic division is used to divide a polynomial function f(x) by a linear factor (x-a). The remainder obtained is the value of f(a). In this case, the remainder is given as 15 which means that f(a) equals 15, not f(15) equals a.
To summarize:
Your friend's conclusion: f(15) = a
Correct conclusion: f(a) = 15
Answer: No, your friend is not correct. The remainder obtained from synthetic division does not provide information about the value of f(15) equaling a.