Was this perhaps a 3 to 6 year old who just thinks it might be fun or was it a 7 to 12 year old who has been wanting to experiment for a long time with crossdressing? We don't even know how she handled the situation with her son, only what she said later to her lunch friends.
Why would age have made any difference? Some of us start really young. The mom sure didn't sound like she'd be very accepting, did she?

While I would be able to understand and support him had he chosen a different path thru his own choice or nature, I would not have at that age encouraged him by allowing him to wear a dress.
You'd have been wrong then. I'm sorry, but that's how I feel. Maybe (hopefully) it would've just been a phase for him. Maybe the poor kid is actually trans, and will not seek treatment now until they've been horribly scarred by testosterone. We'll never know, because mom knows best - even when she likely doesn't know anything, and is apparently prejudiced.