Seahawk, that is not always true. Here's another way to put it:
The number is AXBYCZ. Player 1 picks A, B and C, and player 2 picks X, Y and Z. Together they form a single 6-digit number.
The only winning strategy I could think of for player 1 is to memorize all 6-digit primes that start with one digit (a 7 for example), and pick digit C so there's no Z that can make the whole a prime number. As prime numbers are farther away from each other as numbers get larger, this won't be hard.

http://pdfcast.org/pdf/solution-to-widzisz-s-problem-1

Aww, the "proofs" above are so cute :)

didnt realize it was 0-9

Yea, Piotrek got it right. Glad you liked it :)