Rainbow bridge is a comfort thing for children and adults.
It's a place where animals go when they die and where they wait for their owners. Over the rainbow there's freedom and fields.
When the adult dies, they collect their animals at rainbow bridge on the way to heaven.
I have various beliefs on the afterlife, and it's whatever get one through and helps them. If an adult wants to imagine their beloved pet is waiting for them, then that's ok, I wouldn't ridicule anyone.
But Instagram is no place for chopped up whiskers and fur, and certainly not a place to be discussing this, particularly if her elderly mother gets comfort from this rainbow bridge talk. Keep it private.