Why don't you simply explain to your son that he must earn his own money for the Lego set?
I do...he makes his lego money by catching fly balls...paying off the batter for a “desirable” signature and reselling them on eBay LOL!!
Odd, that she is willing to support Lego, after this act of charity since she disagrees with them so strongly.
If she truly believes Lego was in the wrong in their response to the boy, to be consistent, she can refuse to buy Legos again.