30 percent chance
There are 30 possible numbers that a player can get on their uniform. Out of these, there are 9 single-digit numbers (1, 2, 3, 4, 5, 6, 7, 8, and 9) and 21 double-digit numbers (10, 11, 12, ..., 29, 30).
If no two players can have the same number, then the probability that the first player will get a single-digit number is simply the number of single-digit numbers divided by the total number of possible numbers:
P(single-digit number) = 9/30 = 0.3
So the player is correct that there is a 30% chance that she will get a single-digit number on her uniform.