Usually references to the HOA are buried in the deed/title work and may simply state that the deed holder agrees to abide by the CC&R's. Then it's up to the prospective purchaser to request a copy of the CC&R's, HOA financials, HOA meeting minutes, etc. so that they can be perused before closing. If your mother didn't do that then that's her fault.
I'm not in an HOA community but I am in a "deed restricted" one. It's funny because most of the restrictions don't even apply any more, e.g. everyone will hook up to public water/sewer when they become available (35 years ago they weren't so everyone was on well/septic). There's no HOA to enforce them so if you violate one then someone adversely affected would have to sue you over it. I think there may be five restrictions that are still in effect and most of them are being ignored.
The good thing about it is that there's no way to add a restriction without everyone having to change their deed - which is expensive, and everyone in the community would have to agree to it - so it will never happen.