Rinse it in running water for a couple of minutes (not too long). Let it dry for a day. Put it in the oven at 500 for 15 minutes. Take it out and smell it. If it doesn’t smell chemical-y, it’s fine.
Lye plus vinegar makes sodium acetate, called “moderately toxic by ingestion” in my old chemical dictionary. The LD-50 is pretty high, though, and it is very soluble in hot water, so it should rinse away well. You could soak the stone thoroughly in white vinegar, perhaps by submersion, and then rinse well with hot water from a tea kettle. Rinse until you don’t smell vinegar, dry in a low oven for a couple hours.
Or buy a new one.
Seems easier to just spend the $20
I would not recommend hydrochloric acid-- I don’t know what kind of stone pizza stones are made of, but many stones will be eaten away by HCl. For that matter, some stones will also be eaten by vinegar.
Now I’m wondering about other acids.
Could you neutralize the lye with bird droppings (uric acid) and then heat-sterilize the stone?
Those cheap and greasy refrigerator biscuits are nicely absorbent. You can cover the whole thing and bake a few sacrificial batches to pull up residue out of the stone.
Seems less cheap, for sure. I mean, are we really suggesting lying to the makers in order to score free stuff? Don’t we normally point and laugh at this kinda stuff on notalwaysright.com? 
Table salt is moderately toxic by ingestion with a high LD-50 
I recall an older thread in which a number of people vouchsafed that pizza stones eventually crack anyway, so you have to replace them now and then.
Wait, why even bother with the acid? Last I checked lye(and admittedly I don’t know if that’s what’s in easy off or if there’s other stuff too) is really water soluble.
http://www.inchem.org/documents/icsc/icsc/eics0360.htm
Actually reading that it looks like it’s 3x more soluble than table salt is. If easy off really is just sodium hydroxide all you have to do is fill the sink up with water, put the stone in and wait awhile for the water to dissolve it.
My way:
[ol]
[li]Make a pizza[/li][li]Give it to MIL[/li][li]If she is fine, you are golden[/li][li]If she dies, I’m sure she’ll remember you in some financial way, allowing you to replace the stone.[/li][/ol]
I wanted a pizza stone, but I didn’t want to pay $40 for one. (The Pampered Chef ones cost $40 to $50 depending on size.) I saw the Good Eats episode where Alton Brown suggests using an unglazed paving stone. I think it was terra cotta or something similar. I went to the rock store, and they didn’t have that sort of thing. A guy took me around the yard and we found a roughly-parallelogram shaped piece of slate. Neither one of us knew if it was suitable for pizza, so he said ‘Just take it. If it doesn’t work, bring it back.’ So I got a ‘pizza stone’ free. It works well, but I’ve found it’s easier to use a large cast-iron skillet. That way I get a round pizza.
This cast iron pizza pan works great, too. I don’t bother with a stone anymore. The best part (for me) is that you can superheat it on the stovetop, cook the pizza for a minute or two on the stovetop, then stick it into the oven to finish, to at least mimic some of the charring and flecking you get from a wood-fired style pizza. (This technique is not appropriate to all styles of pizza). Even used the conventional way, the cast iron heats up a heck of a lot faster. I don’t understand that the description says it only oven safe to 400F. I use it at 500F all the time, and much higher temps on the stove.
I just got a square, brick, unglazed “stepping stone” type stone from Home Depot. It’s not terra cotta, just regular brick. Was less than $3. It’s super heavy. The bottom is flat/smooth, so I put it in the oven upside down.
The oven takes a while to preheat now, but it crisps up a pizza perfectly, for like 7% of a Pampered Chef or Williams-Sonoma one.
Did you tell your MIL how to clean it?
Oven cleaner is used to clean cast iron for reseasoning. Cast iron is also porous, though a bit less than stone.
Have done this with pans I use. No one is sick or dying.
Soak the stone overnight, let it dry, bake it and then use normally. 50/50 dilute vinegar is ok to soak in if you are really paranoid.