I had the same problem years ago, and because did not want to take a chance, took it to a jeweler. He got it sparkling bright and it only cost a few bucks. Now, who knows how much it might cost, but I’d ask a reputable jeweler before trying anything myself.
There are jewelry cleaning products out there, but for a one-off job of spiffing up a watch, a little Windex and an old toothbrush or a dish towel ought to be just fine. Your friend’s right to be concerned about getting the works wet - just use enough liquid to dampen the brush/towel.
This page recommends soap and water mixed with a couple drops of ammonia - happily, that’s a pretty good description of most glass cleaners.
At the museum in which I work, we use a product called Nevr-Dull, which you can find at most hardware stores. It comes in a blue can.
It’s a cotton wadding soaked with oil that cuts tarnish really well. Tell your friend to wear rubber gloves, or his hands will get filthy. Rub very gently with a small piece of the cotton wadding. It may take a while, depending on how deep the tarnish goes, but it shouldn’t harm the case of the watch.
I bought one of the ultrasonic cleaners from Harbor Freight for under $50 that does a good job.
I have a gold rolex that I like to shine up before I go out. The untrasonic clean is used when I have more time. I have an extra tooth brush that I put a dab of tooth paste on it and scrub the watch for 30 seconds. People ask me how i get it so shiney and they can’t believe it. I figure if it’s good enough for my teeth it’s good enough for my Rolex.
I have tried this tooth paste on Gold, Platinum and sliver and it works just as good as the ultrasonic cleaning machine.
And I’d be reluctant to use toothpaste on anything made of gold, particularly higher carat gold. Toothpaste is used to polish because it is a mild abrasive. In other words, it scratches the surface. The higher the carat number of the gold, the more pure it is, i.e. the more soft it is, and the more susceptible to scratching it will be, however subtle. Of course, that’s why it’s allied with other metals - to keep it from scratching, but over time, polishing a metal with an abrasive is going to cause it to accumulate some tiny scratches which will cause it to lose luster. I think that the best solutions are the relatively mild chemical ones that remove the thin layers of oxidants and other corrosives. Gold, being pretty much non-reactive, will remain pretty much un-affected by these chemicals.
I have to disagree with using chemicals. We don’t use them at all on metals in the museum in which I work because they sometimes cause pitting.
We use Nevr-Dull on nearly every kind of metal from fine silver to old brass lamps. It only causes tiny scratches if you rub very vigorously, which is why I said to be gentle. I suggest if the OP buys a can of it that he removes the wadding and turns it upside down, taking a portion from the bottom, where there’s more oil because of gravity. Let the oil do the work, not intense rubbing. The rule of thumb that I use is that I don’t rub an artifact harder than I would rub a baby’s skin.
Um well, isn’t Nevr-Dull a chemical? I’m suggesting “mild” chemicals as opposed to abrasives. And I’m saying that gold, being virtually non-reactive, is unlikely to pit. I’d surely defer to your expertise as a museum curator, except that the metals you mention are not gold and that’s the only nit I’m picking here.
It’s kind of a chemical, I guess, but it’s not along the lines of those commercial silver-dips you see on TV. Here’s the specs, which says it’s mineral spirits-- petroleum hydrocarbon. (We use mineral spirits quite frequently in cleaning non-pourous artifacts because it dries quickly and doesn’t leave behind harmful residues.)
I have doubts that the material in question is actually gold, but I can’t say for sure without seeing it in person. It could be gold* plated*, but solid gold (even alloy) doesn’t usually tarnish– it may get a dull film of dirt, but it doesn’t get what we nicknamed “the green meanies” like brass or other metals.
My additional thoughts, based on having purchase over 3000 pocket watches in the last 30 years:
We still don’t know if the pocket watch mentioned in the OP is a gold-filled(plated) one, or one made out of solid gold(which can be of various caratage, usually 10K, 14K or 18K). Pocket watches were never made out of pure gold.
Therefore:
Any pocket watch, whether gold-filled or solid gold will tarnish. A mild abrasive polish will not hurt the value of a typical gold-filled watch, nor, in my experience, a 10K case.
I probably would polish an 18K case a bit more gingerly than something of lower content or GF.
Mineral Spirits by themself won’t clean a watch. It obviously contains a mild abrasive.
A commercial liquid gold/silver cleaner usually contains thiourea. That’s what does the work. By the way, if you use such a cleaner, always use that batch to clean the same thing–all gold, or all silver. Use a fresh batch to clean the opposite, as you may run into a plating effect.