I haven't used Italix pens in the past, so I can't give you a hands-on opinion, but it looks like a fairly sturdy pen. Having the removable button will be nice for both refilling with a converter, and potentially in an eyedropper conversion as well.
Given that it is a British company, the nib will likely be European-style (slightly larger than .3mm (likely .4 or .5mm) which should further improve the likelihood of it writing smoothly out of the box).
In the sense of comparing it to the TWSBI, it will usually have a smaller ink capacity (all converters do) in comparison to the vacuum or eyedropper style pens, though depending on how much your sister writes, that may be enough to last a good month between refills. I typically get between 1 and 2 months per converter-fill on my fountain pens (most converters hold ~0.5ml of ink, though that can depend on the converter) while vac fillers can normally get 2+ml and eyedroppers can get whatever the volume of the pen body is (usually ~3ml, sometimes more).
Noodler's ink is fairly popular out there, and I may have a bottle or two laying around in my ink drawer; but it isn't typically my go-to. That depends on the pen I am using. My Elabo, I love to fill with Sailor nano-black ink (I didn't recommend it in the original post as it is ~$25 per bottle and I was trying to stick to the budget :) ) which should be similar to the Noodler's Eel Black, (though I don't have a good comparison point) in that they are both permanent, water resistant, and should be cleaned well between refills. On my Pilot Preppy 1.1mm italic nib, I have a tendency to go for shimmer inks (like J. Herbin's shimmer inks) as the gold dust in them can give a really cool shimmer there. I will say that in that case, the wetter the nib you have the most shimmer effect you will get, so a F or EF is likely not a good use for shimmer inks. The majority of the inks that I own are Diamine dye-based, which are nice as they are pretty inexpensive (especially if you find a sale or drop) while offering a lot of colors. The only major problem I have with them is that they smear something horrible unless I am using a piece of blotting paper (or a paper towel) under my hand while I write - I am a lefty).