The player did not violate rule 4.2a that covers non-conforming golf balls. The penalty for breaking that rule is DQ.
Instead, the player broke the "one ball rule" - see Committee Procedures chapter 8, model local rule G-4. The player gets the general penalty (two strokes) for each hole mistakenly played with a different brand or model ball (until the mistake is discovered). Intentionally playing a different brand or model penalty is DQ. See the rule for details.