I use Krita for everything, I love it so much. I also won’t act like it’s perfect either, despite it being my most used software by a landslide. Personally my biggest desire now is improved workflow for text editing (e.g. editing text directly on the canvas, being able to box and justify text, vector pathing for text so you can make it bend or wave). From what I understand it is something that is being worked on, and I will be even more indebted to the wonderful folks at KDE once further progress is made on that front.
Using it for porn sounds funny to me given the whole concept of “rule 34” being pretty ubiquitous. If it exists, there’s porn of it! Like even from a completely pragmatic prespective, it sounds like generating pictures of cats. Surely there is a never ending ocean of cat pictures which you can search and refine, do you really need to bring a hallucination machine into the mix? Maybe your friend has an extremely specific fetish list that nothing else will scratch? That’s all I can think of.