As someone who's getting into turning pens specifically to improve existing calligraphy materials, this is especially interesting
The first question to ask him is what kind of calligraphy is he looking at. If it's your foundational and italic hands, those require a
flat nib, something that looks like an italic or stub. You can find those separately and just stick 'em onto the pen, most of the sections are made friction fit.
If he wants pointed pen calligraphy, he has two options. The first is expensive. Get a nice gold nib, 14k or 18k, and then get a custom grind done by someone with serious nib experience. Mortishaw, Binder, the like. The other option is to stick on a holder that will allow him to insert a variety of dip nibs, like attachment (though it's an oblique holder, that can be attached straight as well).
I definitely do not recommend getting a dip nib stuck on there permanently, as they are cheap and meant to be replaced as soon as they lose their fine point. Here are some examples (my work) of both broad pen calligraphy and pointed pen.