David
Fountain pen 'refills' are usually referred to as 'cartridges'. You and your wife need to first understand the difference between 'standard international' cartridges (or more correctly, 'standard international short cartridges') and proprietary cartridges. The differences relate to the dimensions of the cartridges, most importantly, the dimensions of the female portion of the cartridge that must match exactly a nipple in the pen section.
Proprietary cartridges are those that are made by a pen manufacturer for exclusive use in the pens they make; they generally won't fit pens from other manufacturers (although there are a few exceptions). There is a fairly short list of manufacturers who choose the vertically-integrated approach and make proprietary cartridges: the list that I have includes Cross, MontBlanc, Platinum, Aurora, Faber-Castell, Lamy, Namiki, Parker, Pilot, Sailor, Shaeffer, and Waterman.
By contrast, other manufacturers design their products to use essentially universal cartridges, more generally called 'standard international short cartridges'. (There are also 'standard international long cartridges', but they are possibly misnamed because as far as I know, only one manufacturer makes them and they fit only one brand of pen - Pelikan - so they really aren't standard standard, but that's a diversion).
That said, the first thing that is important to your situation is that
pens made from kits are all designed to work with standard international short cartridges, and because Waterman cartridges are proprietary, they won't fit your wife's pen.
You mentioned that you were unable to install a cartridge that came with another pen kit in your wife's pen. Obviously, while the dimensions of standard international short cartridges can be standardized, there can still be differences. Most pen kits and components come from China which also produces a lot of cartridges, some of which are good, and some are junk. It therefore wouldn't be a surprise to find variations in the quality of cartridges supplied with kits.
But that said, it is true that sometimes it takes a bit more force to install a cartridge.
So where does that leave you. I think you have two choices:
- There are many very good inks out there from reputable suppliers that are packaged in standard international short cartridges. Brands include Diamine, Private Reserve, Pelikan, Jacques Herbin, Montegrappa, Monteverde, Kaweco, and many others, and those manufacturers offer a dazzling variety of inks to choose from.
- It is also possible to purchase a device called a 'converter', or sometimes a 'pump' that fits into pens that accommodate standard international short cartridges, and that can be filled using bottled ink. Most major pen retailers sell standard international converters, as well as converters to fit pens that are designed for proprietary ink packaging. Converters hold a bit more ink than standard international short cartridges, but the process of refilling can result in stained fingers (a point of pride among fountain pen fanatics!), but rubber gloves prevent that problem for those who are more fastidious. The selection of bottled inks is even broader than the selection packaged in standard international short cartridges - AFAIK, every ink packaged in cartridges can also be purchased in a bottle, but there are probably five times as many more inks that are only available in bottles including some small boutique brands. So what that means is that if your wife really likes Waternan ink, the option is there to use bottled Waterman ink in a converter. Alternatively, purchasing a converter opens the door to hundreds of other inks to choose from. And while ink is actually very inexpensive, ink purchased in bottles is even less expensive that ink packaged in cartridges.
Finally, let me point you to some suppliers. First, pen kit suppliers often sell converters as accessories. I know that PSI offers converters at two price points - I've used both, and I prefer the more expensive version, but I've had success with the cheaper model also.
Second, for converters, cartridges and bottle inks, my suggestion is Goulet Pen Company, .https://www.gouletpens.com. The founder and President, Brian Goulet, started out as a hobby pen turner. While Goulet does carry some proprietary pens and cartridges, they don't carry Waterman products. However, there are other very reputable suppliers on the internet - Dromgooles, Fountain Pen Hospital, Pen Chalet, Fahrney's Pens, Goldspot, etc.