I'm trying a simple recipe for ginger beer (Method 1 here). Would using ReaLemon be ok instead of an actual lemon? I understand that preservatives in an apple juice can harm the yeast when making cider, but will this small amount (2-3 tablespoons) of lemon juice make a difference?
Using ReaLemon could negatively affect your ginger beer, but it is hard to tell the effect without knowing the concentration of free sulfite (SO2) in ReaLemon.
ReaLemon's label says it contains sulfite and the manufacturer says that any of their products that mention sulfites "contains 10 ppm or more of sulfites". After a short period of tolerance, Saccharomyces cerevisiae (aka brewer's yeast aka baker's yeast) were irreversibly damaged by low concentrations of sulfite according to this study. Free SO2 is widely used in the winemaking industry to inhibit yeast.
So it is difficult to tell the effect when we don't know the concentration of SO2. Home winemakers shoot for 25 to 50 ppm of "free SO2" to inhibit unwanted fermentation. To be safe, let's estimate that ReaLemon is on the high end of that range. (In fact, in the EU, the maximum level of sulfite is 160 ppm for red wine.) Your recipe calls for the juice of one lemon, which equals about three Tbsp. or 1.5 oz. of ReaLemon. It looks like the recipe is for 24 oz. of ginger beer, estimating from the botle size in the recipe picture. So if you dissolve 1.5 oz. of ReaLemon containing 50 ppm of free SO2 into 24 oz., you end up at a concentration of 3.75 ppm of free sulfite.
Some commercially-made wines sent for lab analysis contain as low as 5 ppm of free sulfite, which is apparently deemed enough for preservative effect, but not enough for long-term aging. At low levels of 5-10 ppm, SO2 delays the onset of fermentation, but later speeds up the multiplication of yeasts and their transformation of sugars [Peynaud, 1984].
In short, we can't definitively answer your question without knowing the sulfite concentration in ReaLemon. Why not just use a real lemon since there is doubt?
EDIT: The recipe size is 2L, as @mdma points out, so assuming ReaLemon has 50 ppm of sulfite, then the final concentration in your recipe would be a very low 1 ppm.
The recipe is 2 liters, with 1 lemon's worth of juice (15ml) diluted. With that much dilution, the sulphate content in ReaLemon would have to be absurdly high to permanently affect the yeast.
If you can get hold of a lemon fruit, then use that. It's not been processed and you will get much better aromatics in the final product. But if you really can't get a lemon, then use the processed substitute (ReaLemon.)
I make ginger beer with ginger beer plant. I tried using lemon juice with preservatives exactly once and ended up killing my plant, and this was in similar concentrations, about 3-4 tablespoons in a gallon of juice. Ginger beer plant is a combination of yeast and bacteria, not just straight brewer's yeast like you are using, so maybe it has less tolerance for some reason, but I still would not risk it.
But, if you DO try it, make sure to report back your findings, I'm interested to hear what happens.