Personally, I think the utility of stem shifters depends a lot on the handlebars you're using and how you ride. If you have drop bars and ride in the drops, then they're probably not a good choice. On the other hand, OP's wife rarely rides in the drops. Since she's usually on the tops, stemmies would be more suitable. I have a bike with inverted Nitto Albatross bars, and stemmies work wonderfully on it. (I might be using barend shifters, except that I already have inverse brake levers mounted in the bar ends.)
If you want stemmies, you do have options.
1. It is possible to purchase shims that will enable you to mount DT shifters with a 28.6mm clamp on a 22.2mm quill stem. I actually have one, but I can't for the life of me remember where I got it. In any event, using it wouldn't be my preference, because it looks kludgy. For that reason, it's just sitting in my parts drawer.
2. It's also possible to convert DT shifters to stem shifters by mating the DT shift levers to a stem shifter clamp. I know because I've done it. The advantage of the stem shifter clamp (in addition to being the right size and not needing a shim) is that it has built-in cable stops. A few old school clamp-on DT shifters also have these, but most DT shifters don't. The problem with this method is that you have to make sure that the DT shift levers and the stem shifter clamp are compatible.
3. The easiest solution, by far, is simply to buy a set of stemmies. Because they're not very popular, they can be found on ebay for very little money. I'd say that friction (my favorite) are the most common, but indexed are available, too. If you can get a set of Suntour Power ratchets, that would be my recommendation, but there are others that will work just fine.
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Steve