hello! you are probably thinking of either Scratchapixel (the tutorial I was following), specifically their article on rasterisation, but maybe also my Building My Own Rasteriser series, which is the slightly more detailed notes I wrote while following along their guide some years back.

Scratchapixel is a fantastic resource, I highly recommend it. And a simple rasteriser is also a great way to learn what's going on under the hood in graphics programming, it's a great project to do, good luck to your friend!

Other useful articles and books you may find of interest:

Vulkan tutorial - in case you want to move beyond toy software rasterisers and use an actual hardware graphics API, but you don't want to just abstract it away using a library, this is where to learn how to drive Vulkan.

Physically Based Rendering: From Theory to Implementation - the definitive textbook on how to write a raytracer, available free online. A lot of the theory (microfacets etc.) will also be extremely relevant for surface shaders in a rasteriser.

Inigo Quilez's website: Inigo Quilez is the cofounder of Shadertoy, and probably the world expert on raymarching signed distance fields. There's a huge amount to explore in there if you're interested in that kind of technique and all sorts of other weird graphics tricks.

A list of frame breakdowns/graphics studies, another one - I remember finding another site with these but I can't find it right now. In any case, games usually use quite complex deferred rendering techniques, with multiple render passes combined in various ways. One of the best ways to learn about this is to use a tool like RenderDoc to break down all the draw calls in the frame, and see exactly what it does. So have a look through those pages to learn a bit how games are rendered!

game art tricks - I actually just found this one while writing this post but it's good! primarily visual effects focused, lots of very ingenious approaches to rendering things cheaply.