Sure it'll run on three. It'll be rough and noisy and I wouldn't keep it running that way, but it will run. Who is the "they" which isn't sure how to get the 4th cylinder running?

Sorry, they is the curb side mechanic. He might not have all the diagnostic equipment of most shops, but my friend cannot afford a nice shop for it.

As said above, yes it will run on 3 but it will be rough, noisy, and will have drastically reduced power output.

I knew of a VW 113 running on three cylinders due to a burned exhaust valve (no compression in that chamber), but it was smooth - for some reason.
Most four-bangers will be very rough when running on three - try pulling one plug wire, and then pull one more - very few will run at all then..

Additional Information: Do not pull a plug wire on a running engine; Do not remove a plug wire (and leave it dangle), and then start the engine. Always short the removed plug wire to the engine. An open circuit in the ingition's seconday circuit will overstress the ignition systen voltage-wise. Some modern ignition systems may have over-voltage protection built-in, but they are getting undue voltage before this kicks in. Why take a chance on damaging your ignition system.

So what has the curbside mechanic tried to get the fourth cylinder running? Maybe the folks here have some other ideas.

Just 4 fun I'll guess its a spark wire broken some distance back from the plug.

They say it was was the injector for the one cylinder that
was not working. I hope that's it and they can fix it!

I have an idea what she should do, and it's go to a real shop, not a "curbside mechanic". Most of the people who will read this have no idea what it takes to equip a shop, and train the techs to work on today's cars. The facts are that the "curbside" guys don't know either so they try and do things that they are really not capable of, which often ends up simply swapping parts until the car owner runs out of money, or they happen to guess correctly and actually manage to fix the car. But meanwhile the curbside guy, is he paying his fair share of income tax? Is he paying mercantile tax? Unemployment? Insurances? How is he disposing of the hazardous wastes created by servicing a car? The list goes on and on for normal expenses that a shop has that these people do not. But above all of that if he was really any good at fixing cars he would be employe'd at a shop making a decent living. Ask yourself why isn't he working in a shop? If he's not good enough to do that do you really want him touching your car?

So why with all of those facts does someone even consider these kinds of attempts to have the second most expensive investment that a family owns serviced? An engine missfire should be easily diagnosed in an hour or less, without replacing any parts! Why would they even look at a car with a problem that they dont have the tools and equipment necessary to do the service correctly? If you want a definition of a rip-off you have one right there. Attempting to service a car without proper tools and training is a rip-off. Sure they may try and do it "cheap", but price is not the real measure of honesty. In fact in any other job/business the more training and experience an employee has the more money they expect to make. Why should repairing cars be any different? The more experienced the shop/techs are, the more equipment they have and use, it only makes sense that they are going to be more expensive on the surface to sustain that level of expertise. But where people drop the ball on the expectation is the shop that does it right finds the solution to all of the problems quickly and efficiently, and that actually results in lower service costs because there are no unneccessary parts replaced.