I used to have only Intel systems, until my latest 2 systems built.
Besides gaming, I used multi-threaded apps, Ryzen 7 CPUs are excellent at multi-threaded tasks and good at running applications that take advantage of multiple cores.
With Intel chips I was getting a little bit of better performance in gaming but losing out in heavy multi-threaded settings.
If you build a new system for your fiancee she might not mind going with AMD.
You don't have to go with the 8700k, most gamers are better off with a mid-range CPU (i5-8500) and spending what they've saved on a better graphics card.
Any component that works on Intel 300 series chipset will work on AMD 400 series chipset.
AMD and Intel are producing CPUs that are are neck to neck of one another on nearly every front (price, power, and performance). Like the Ryzen 2700x competes with the 8700k.
Intel chips tend to offer a little better performance per core, and AMD compensates with more cores at a lower price.