After a suitable amount of downtime, I would allow them to use the guide's proficiency bonus (+3 or so) in place of any proficiency bonus they may otherwise have for any associated skill checks. So if the player is untrained in Arcana, and a monster related question would call for an Arcana (Int) check, they could use 3+ whatever their intelligence bonus is. If they know more than Volo does (higher proficiency bonus) then the guide would be of no help.
