Ok - get more details from the doctor - get the MRI report and a copy of the MRI.
Likely, she has an arachnoid cyst or subarachnoid cyst and yes, the lore is that people are born with them and that they do not do anything. But already you are sensing that something is not right. I think you need to persist in mom-sense and dump the *#$(*$ who said go back to your PCP and ignore it and instead, speak to your PCP about your options.
It may be that you can wait for a while, but while these are fairly common (they develop in the lining around the brain, if that is what she has), she may still need competent treatment.
I don't have any advice to offer just some sympathy, I can't imagine going through what you are :(