Only if you agree to retroactively allow him to play that card first before the actual swap. We're pretty flexible when it's actions on one's own turn in allowing changes (so we would have allowed it), but technically as soon as your hand size is too large you must IMMEDIATELY discard down to the legal size.
It doesn't need to be retroactive. You're correct that once the card is received, the player is over the hand limit. But, the rules allow you to either discard or play event cards to get back down to 7 cards.
For specifics, it can be found on page 10, under the Share Knowledge action.