Linux

I've decades of computer experience been working with 3D on and off for years. IMO, there's really no magic bullet solution that will likely offer the level of quality and detail he needs given his experience level. There are of course other 3D modeling softwares out there, but for something like custom jewelry I don't expect they'd be much easier to pick up than what you've already tried. However, a combination of tools might do the job better for him:

If he's good at math, then you might instead consider a mathematical based modeling software such as OpenSCAD to build components by formula/code, and then maybe Blender to put stuff together. OpenSCAD has generally been my go-to fit designing parts etc with known dimensions.

A third option might be a good 3D scanner, turntable, and modeling clay etc. He could create the component pieces as real models, scan to a computer for, then touch up, combine, or modify the scan results to build a final product.

Last suggestion, even if he won't be printing the final product, it may be a good idea to pick up a small printer (depending on the size of the jewelry) that does decent detail, so he can do mock-ups and see what things should look like. Even given the cost of a printer (which for a decent small one shouldn't be onerous), catching a flaw in a model done in plastic is probably going to save you money in the long run if you catch it before it gets to the silver and gold stage :-)

As I said, I don't know much about designing jewelry, so I'm probably missing some subtleties. I've had a look at Jewelcraft, and it looks like 90% of what you need to know is standard Blender. Once you start adding components to your asset library it looks like it becomes quicker and easier too. Blender can certainly handle settimg the dimensions on parts, and exports to STL ready for printing. If Rhino feels like a better fit though, they do have a 90 day free trial, so you could see if it works under Wine. It'll have just as much of a learning curve as Blender though, and your likely to find more help for an open source system like Blender.

I'll add my +1 for FreeCAD. It can export to stl (you mentioned that in another comment). Though, if it is a good fit for your father's needs is highly dependent on the kind of designs he's trying to achieve. FreeCAD is good for "geometric" designs, not so good for "organic" shapes. As any tool, it comes with a learning curve.