Wait until it stops and clears and has been stabilized before adding the sugar, then add until it tastes good to you will depend on the acid content and tannins present. If sugar was added now and the yeast is high alcohol tolerant it will just ferment, since its wild yeast there is not any info on the tolerance level of it.
Your neighbors method will work to stabilize, adding the enough sugar until the yeast poison themselves with alcohol, take gravity readings and add some more sugar and let it ferment out until it stops. Make the sugar into a syrup first, if you add it straight as granules co2 comes out of suspension and you will have a wine volcano.
Wine conditioner is glycerin, sugar and potasium sorbate, its used after your wine is done and must be used in the recomended amounts or there will not be enough sorbate to stop the yeast, it wont stop a active ferment either it just stops the yeast from multiplying any more and then eventually they get old and die off slowly.
If there is a malolactic fermentation going and sorbate is added you will have a strong germanium odor to it and will be undrinkable, so sulfites must be added to 50 ppm to prevent MLF. Since it is wild yeast i would be warry of adding sorbate, yeast are in there that means there are more bugs than that got in
Before you bottle make sure it is clear, pull a sample with a wine thief and go into a dimly lit room with a flashlight shine through it, the beam will light up and reflect off anything in the wine, if its clear shouldnt hardly see the beam at all.