Q: Apple did not return "defective" part

My son purchased a mid-2012 13-inch MBP and, in January of this year, upgraded the RAM to two 4-GB sticks from Kingston.  A few momths later, his MBP gave three beeps and then shut down. The machine was under AppleCare, and so was sent in for repair.  Apple determined that the logic board was faulty (was not allowing the RAM to boot), and so replaced It.  The same sympoms reappeared a few weeks ago, and so once again he sent in his MBP to be repaired. 

He received his laptop a few days ago.  This time, the repair notes stated that the issues were caused by corrupted RAM, rather than a faulty logic board.  They replaced the two 4-GB sticks with two 2-GB sticks at no charge; however, they did not return either of the 4-GB sticks.  While he certainly understands that RAM can fail, he would have liked to have been able to independently verify if what they are saying is true.  He is on a tight budget, and so replacing the RAM is a big deal for him.

He spoke to customer support earlier this morning, and they basically told him that he is out of luck.  He has searched through Apple repair policies and could not find anything that says that they are required to return DIY parts.  Before giving up the ship, he asked me to check the collective wisdom of this group and see if he has any recourse.

Thoughts?

Thanks.