Is there a printed guideline somewhere about this 90 day limitation?
No, but B2 visa is given for people with non-immigration intent. People declare their intent on the paper while applying. Filing in very short time may be proof that immigration intent was already established.
However, Nav102 can still breath normal as USCIS is lenient towards USC's parents who are considered immediate relatives with no visa availability restrictions. There is a 2% probability of USCIS pulling Nav102's parents, so they better keep their statement ready on how they changed their mind so fast upon reaching USA.
On one of the other website I was advised that it is advisable to wait for 60 days. My neighbor filed for his mother within 30 days and nothing happened - not even an RFE. If you look at it the other way, sponsors really start gathering all the documents (medical exam, birth certs, employment letters) etc. before 90 days. By looking at the dates of these documents one can easily notice the intention for AOS even if you may actually date the I-485 differently.