Is it an official rule that interference in Monopoly requires paying $50 to free parking?
That is a house rule as there is nothing in the base rules about communicating with other players and there is also no such thing as paying free parking. Honestly it is a rule that hurts less experienced or younger players as it discourages others from helping them play when other players may try and take advantage of their inexperience/age.

While the rules do say that you can't collect rent if you don't notice before the next two players throw the dice it does not say that other players can't remind you to collect the rent you are owed.

Rules

PAYING RENT…


  When you land on property owned by another
  player, the owner collects rent from you in accordance with the list
  printed on its Title Deed card.
  If the property is mortgaged, no rent can be collected. When a
  property is mortgaged, its Title Deed card is placed face down in front
  of the owner.
  It is an advantage to hold all the Title Deed cards in a color-group
  (e.g., Boardwalk and Park Place; or Connecticut, Vermont and Oriental
  Avenues) because the owner may then charge double rent for
  unimproved properties in that color-group. This rule applies to
  unmortgaged properties even if another property in that color-group
  is mortgaged.
  It is even more advantageous to have houses or hotels on properties
  because rents are much higher than for unimproved properties.
  The owner may not collect the rent if he/she fails to ask for it before
  the second player following throws the dice


Free parking is just a safe space on the board and you don't gain anything from it.

“FREE PARKING”… 


  A player landing on this place does not receive
  any money, property or reward of any kind. This is just a “free”
  resting place.