I recently invited my friend and her family to my son's birthday party. They brought my son as a gift: a pre-paid MasterCard gift card. Now, after couple of months, my son tried to use the MasterCard but discovered that it's empty; there is no money on that card. We already have sent them a thank you card. How should I let them know about it or should I just leave the matter as it is?
I think you should tell them. I'm sure they didn't mean to give your son an empty card. In fact, there very well could have been a glitch in the card or the machine that didn't register the money they paid!
The only way to find out is to let them know. Tell them just what you told me: that your son just went to use the card but it was empty. You think there must have been a mistake and you don't want their money to be wasted.
Make sure to stress that you're not after the money; you only want them to get back what they paid. Hopefully they got a receipt and can sort it out.