The risk is as close to zero as it gets.
What country are you in? Syphilis is uncommon in most developed countries, like the US, UK, Canada, most European countries, etc.
A quick lick like that isn't going to transmit anything, and unless the person had a syphilis sore on the end of their tongue, there is zero risk. If they did have a sore on the end of their tongue, it was so brief that it would be very, very unlikely to transmit.