Your friend needs to take the blood thinners. The reason being is that people with serious valve insufficiency or regurgitation can develop blood clots which can lead to a stroke. I have atrial valve insufficiency but so far I haven't had to take blood thinners or needed surgery. My brother takes Coumadin because a few years ago he threw a clot and stroked out.
Aortic insufficiency is very serious and he needs to follow doctor's orders. He needs to ask his doctor about the effect the blood thinner will have on his colitis.