Thank you to everyone for your advice. I have good news...

Yesterday morning my mom told me that my friend's mom reconsidered her decision and told her son about her condition. I didn't get a chance to talk with her about it, she decided on her own, thankfully. My friend and his family will still be going to Disneyland.

I didn't want to go against her wishes (if I wasn't uncomfortable with it I wouldn't have asked for advice here ;-)) and now I don't have to.