If she's only interested in a clear/white stone you could consider a white topaz or white emerald, they look very similar to diamonds! Since these are natural stones they will hold their color and clarity nicely. They will not be as tough as a diamond, but I would think they would hold up well enough assuming she is moderately careful, white emeralds in particular are quite strong.
I tried to get an emerald ring as an engagement ring and the jeweler told me that the stones are so soft that they're usually not used in rings. Do you think the jeweler was blowing smoke up my ass, or is this true?
This is true. I work for a very reputable jewelry store. Emeralds (which is the green version of the stone beryl) are very soft, abrade very easily, and are generally not recommended for daily wear. At least if you want them to stay nice-looking.
CZs will also abrade and generally look poo after a few years of daily wear. They are so cheap though, you could have it replaced periodically.
I don't have experience with Moissanite, we typically only deal with genuine gemstones.
White sapphires are a good choice if you do want a natural gemstone. They are a 9 in terms of hardness. They WILL abrade, I have seen abraded sapphires many, many times. Diamonds, as a 10 on the Mohs scale, are about 4-5 times harder than sapphires/rubies (which are different colors of the same stone, corundum). Diamonds really will outlast any other gem in terms of abrasion resistance.
To someone who knows what they're looking at, white sapphires really don't look exactly like diamonds. Moissanite might be closer in terms of appearance.
If you buy online, you really don't have anyone to take it back to in the event something happens to it, or it needs periodic maintenance (such as if you drop it down the garbage disposal, or break a prong, or even just the stone needs tightened). Mine doesn't, but many jewelry stores do charge extra to do work on a piece that was purchased elsewhere. Either that, or refuse to work on it at all. Keep that in mind.
If you are working with a local jeweler, ask about their pricing for re-setting if you choose to get a CZ. The CZ itself shouldn't cost you more than 15-20 bucks, even for a big one. You may find it cheaper and more attractive in the long run to get the CZ and replace it every 5 years or so. Anything else you buy really is going to abrade if you wear it 100% all day every day, and will cost a whole lot more to continually replace. Maybe you don't care if it does abrade, that's totally up to you.