What to say to a non-Muslim if he sneezes and praises Allah

"May Allah guide you and set right your affairs."

For a non-Muslim who sneezes and praises Allah, one says this prayer for guidance rather than the usual "May Allah have mercy on you" that is said to a fellow Muslim.

[at-Tirmidhi 2739; Sahih Sunan at-Tirmidhi 2739]