When they get to 16 DVPs, a resolution will appear in the World Congress votes that will allow you to vote to give or take away 2 points from people.
Just to clear this up, the resolution to add or take away DV points to players appears when the world enters Modern era, not when someone gets to 16 DVPs.
And ganging up on the DV leader can start already at 14/20 points, though it is less likely. At 16 it is pretty much guaranteed that you won't be able to outvote everyone else, so just put one single and free vote against yourself.
How to stop AI from winning DV? Winning earlier usually works
If you let the game go on well past t300, AI is likely to stumble upon some kind of victory, including DV, although the most frequent type for the AI is SV.
Just paying a little more attention during WC votes can bring you enough DV points to reach 16/20 and then you make the finishing slam dunk with the SoL which Shah Jahan kindly completes in a single turn. On standard settings SoL is usually ignored by the AI and is left there for the player to pick it up, unless you put more AIs on the map and it is water heavy.
During voting sessions you can leave the voting screen and go to the map to check things out, which is handy. All your votes already made will stay when you come back. And you may want to check things out when a resolution such as World Ideology comes up to decide what government gets an extra policy card. So you just go and check what government form is adopted by the majority of the AIs and then come back to see if they have plenty of DF to back it up and then put the single free vote for that form of government.
Some resolutions are guaranteed wins, like Border Control Treaty, where you will always win by putting just 2 votes for yourself to have culture bombs on your districts.
Then there is the question of having enough DF to make influence. You can try and buy up DF from the AI (although most players will do the opposite), but they will sell no more than 20 DF per turn, so this is pretty tedious. And selling will stop entirely when you approach the fateful 14-16/20 limit.
Being in Monarchy and having Renaissance walls everywhere is much easier, those bring you +2 DF per turn, usually that's quite enough, so if the resolution of Urban Development Treaty comes up, city centre option is quite likely to win, and when paired with Limes you can have those walls up in very short time.
So paying a bit more attention during the votes and winning WC competitions, and also winning Aid Emergencies, if any come up, can lead you to 16/20 with a rather low effort and give you the DV somewhere in the range of t240-290, which is very safe win margin.
But in this particular case when you let AI to have 19 points might be a bit tricky. Make sure she does not win any WC competition and there's no DV bringing Wonders left for her to build (Mahabodi, Potala, SoL). But then, there are still a couple of DV points in the tech/civics trees which you can do nothing about. If she reaches them, well, too bad.
