The only restriction for where flyers can land, from page 71, "It may not finish the move on top of another unit or in impassable terrain." There is no restriction on forests in the flyer rule section and forests are not impassable.
I can find no rule in the terrain section forbidding flyers landing or charging into forests, but it does say that, on page 119, "Models with the flyer special rule that begin or end a flying move in a forest must take a dangerous terrain test". Again nothing forbidding flyers to go into a forest, but a dangerous terrain test must be taken.
Terradons have the forest strider special rule so would not in fact have to take this dangerous terrain test.
Ripperdactyls do not have have the forest strider special rule so would have to take the dangerous terrain test.
I don't see how this is open to interpretation in all honesty. If they push it again you have to ask them to find the rule in the rulebook saying you can't do it and you can point out these two pages to them.