In 1915 the Turks and the Germans sent a diplomatic mission to Kabul to try and sway the Afghan king to join the central powers and attack British India. The Plan was for Afghanistan to Enter onto the Turkish side and invade India, hopefully creating nationalist feelings in India and in turn cause India to rebel against British rule. In OTL this mission led to the Afghans recieving a huge payment and arms provision for invading India.
According to the all knowing and all powerful Wikipedia Britain, while officially controlling Afghanistan's foreign Policy, really had little power to enforce it. Also at the time Britain considered Afghanistan to be the only country capable of Invading India.
What happens if Afghanistan accepts and invades India. The Afghans would likely have much more soldiers than in the Third Anglo-Afghan War because one of the Rallying cries by Mehmet V was that WW1 was a Jihad against the Allies and he was, for most, considered to be the Caliph. In the Third Anglo-Afghan War the Afghan troop levels were around "50,000 men in 21 cavalry regiments and 75 infantry battalions, plus 280 modern guns and up to 80,000 tribesmen" Straight from Wikipedia.