Subject: Poggibonsi/San Gimignano
The best way is the motorway Firenze-Siena, from Firenze Certosa to Poggibonsi Nord (if he chooses the faster way), or the Cassian Road ( S. S. 2) (if he chooses the more panoramic way).

In both cases, he has to enter the northern part of Poggibonsi, straight on until the bridge, then right following the indications to San Gimignano.

As regards means of transport, take note that travelling either by train or by bus, your friend has to stop in Poggibonsi just to wait for a bus to San Gimignano.

That's as far as I consider the best way, Flavio