As other have said, please do an SSD swap.
If it's "unbearably slow" that is an indication of drive failure especially on old boot drives. Linux will not fix this.
After that, Cinnamon if they like windows. Gnome if they don't or don't care.
Pop os is a great "fire and forget" OS for normal users. I work in a computer shop and have seen tons of not-knowledgable people run it without issues.