If you have flat feet, in the case of your friend, the good news is that it is not a "death sentence" for a good running career. Especially with today's top of the line shoe brands and models, as long as you find the right shoe, your flat really will not set you back at all.
The foot's arch is the body's natural shock absorber when it comes to high impact sports and cardio (like running). As the foot lands on the trail or pavement with each running step, it creates a lot of stress on the foot and lower parts of the leg. The arched tendons in the bottom of the foot are there to help relieve this stress and reduce harmful impact. But people who have flat feet have "fallen" arches, so the tendons on the bottoms of their feet are not as arched and taut, and do not absorb high impact shock as well or as efficiently. Flat feet are also indicators of over pronation, meaning because your fallen arches are weaker upon impact, they turn over on themselves and roll the ankle inward. This can cause a myriad of foot, ankle, knee, and joint problems and pain down the road if left untreated.
For a complete list of some of the best shoes and brands for people with flat feet, check out our great list found here. (And if you already have a pair of shoes you love that are not on this list, you can check out our list of some of the best insoles for flat feet found here.) In short, you will want shoes that have a lot of support, and may want to consider ones that have soles designed and shaped upward into a false arch. And look for shoes that have higher tops that surround the ankle, to help prevent over pronation and provide stability to your foot's tendency to roll inward. The best shoes for flat feet will be heavily cushioned, extremely durable and stable, and provide motion control.