Did I make a mistake by asking my friend to promise to pray during the accident, and by urging him to perform the Sunnah prayers, and how can I encourage him to do them indirectly?

May Allah reward you for your eagerness to guide your friend. There is no harm in commanding him, in the name of Allah, to observe the prayers if Allah saves him, because prayer is one of the obligatory duties that Allah has entrusted to His servants. As for the Sunnah prayers, gradually impress upon him their importance. You can make him love performing the prayers by reminding him of their virtues and the reward he misses out on. Strive to invite him with gentleness and kindness.