Let total cost be "c".

From the original we know that the cost is being evenly divided among three friends. So, the amount that each friend pays is given by the expression c/3.

(2) tells us that when we add another friend to the division of cost, each of the original three pay 15 less. So, now each of the original three pay c/3 - 15, so:

c/4 = c/3 - 15

from which we can compute "c" (not that we will because this is DS), and so (2) is sufficient.

Because (1) gives us no info about actual cost, (1) is insufficient.