Sure you could add another leaf to the pack, which is basically doing the same as any stop-gap measure, in the end the only real cure would be to replace the fatigued springs with a new set. Even having springs re-ached will weaken them causing failure in the future. If they were not old and fatigued I would suggest adding leafs but a fatigued set will only fail in the future. It's a crap-shoot with any of the fixes on fatigued springs but the easiest, least expensive, least work, and quickest is a shackle lift. Now if he decides to go that route and add a leaf he needs to remove the flat load leaf and replace it with a long load leaf this will help to avoid a harsh ride.