I completely understand wanting to be the "goto" sitter. My dd1 left a family camping trip a day early last year to help earn this status
it's been extremely fruitful for her. Now, a year later, she can offer her sibling as a back up since he watches them too. (This is her voice teacher and ds sometimes babysits during dd's lessons
)
A warning bell just rang when u said they r restricted from those types of social activities. Trying not to judge and treading lightly because everyone's family rules are different ... but in this situation if most communication is through the parent, If u allow her to cancel, I would make the explanation myself even if your dd tells her friend. That's all I feel comfy saying in a public forum.