Koi and goldfish may interbreed, the resulting fry are usually somewhat undesirable. The true answer to your question really depends on what zone this person lives in. The fish can be overwintered in cold/freezing zones if the pond is heated, MANY folks opt to bring the fish inside for the winter.
As for pond-mates, heck, when I was a kid my little sister's guppies went on a breeding FRENZY (all fancies). It was during an El Nino year, and we ended up netting a bunch of 'em up and putting them in a pond that had developed because of the rain. They lived as long as there was sufficient water (which was gone by next spring).
In our small pond (250gals.) we have one awful koi (not entirely malformed, but to my eye close enough) and several pretty comets. What we've really put our effort into are the plantings, with lotus, papyrus, water lettuce, and some kind of reed.. along with some others I'm drawing a blank on.
Tell your co-worker to be prepared for the summer ulcers (furunculosis). Steven Pro had linked me last year to some sites outlining treatment for it over on wetwebfotos.com/talk, but I've since lost it. S/he should be prepared, as it's an ugly disease. Our koi pulled through it last year, though we'd thought for SURE it was dead. Come spring, out popped this fish more than twice the size it had been! (This pond is in SoCal.)
