First, it would be a small claims court kind of thing... if they could make the case, based on their valuation at $300.
I think if the games were borrowed while
BM had custody, then she would be the responsible parent.
Next, the value of the game is NOT the replacement value, but the value of the games on the day they were damaged (effectively the day the games were returned, because they are alleging damage and you allege there is no damage that was not originally there.)
AND the BF/cousin would have to GIVE UP the games in order to get full "actual cash value" ..... OR ...... he would have to settle for some amount smaller to keep them, and take a reduction in value for the games as they currently are...the difference between what they would be worth undamaged and the value of the now damaged games. Don't forget the burden of proof is on them.
I.e., game is bought at $50 six years ago. BF used the game for 4 years.
SS borrowed the game 2 years ago. SS returned the game. Damage is allegedly found after the return on this 6 year old game. The game on the date of loan was 4 years old and the loaner EXPECTED the game to continue to age while being loaned to SS (shouldn't get compensated for the devalution based on age.)
So go find a USED game of the same sort at a game store and take a picture of the game and price. After all, SS borrowed a USED game, not a new one. Keep the information on these items in your files and use if necessary. (Don't make any special trips for the information, just drop by when you are already in the mall.)
I doubt that the ex and BF/cousin will really go after this, but some people are so blinded by their fury that they get stoopid.