Dark Dragon
Explorer
IIRC, the bonus spells are lost. There was a discussion about that question IMG and a player found a rule in the PHB (don't ask me where, I don't have the books at work...).
If the druid changes back, he won't get the bonus spells unless 8 hours have passed after loss of these spells and a prayer at the right time (normally in the morning, but that depends on the druid's patron deity).
So the druid should choose spells with a long duration as bonus spells (e.g. GMF, Endure Elements, Blind Sight, Delay poison for his wizard buddy, Spikes, Energy Immunity,...) and cast them before using his first WS for that day.
If the druid changes back, he won't get the bonus spells unless 8 hours have passed after loss of these spells and a prayer at the right time (normally in the morning, but that depends on the druid's patron deity).
So the druid should choose spells with a long duration as bonus spells (e.g. GMF, Endure Elements, Blind Sight, Delay poison for his wizard buddy, Spikes, Energy Immunity,...) and cast them before using his first WS for that day.