Umm...you didn't want to make him uncomfortable in front of the doctor...so instead, you posted it to the global internet..? The World-Wide Web..?
This is why men don't live as long as women.
Fortunately, Viagra use is so old hat and commonplace that I don't think anyone really cares.
Unfortunately, the only person who can gauge the safety of Viagra for him at this point would be his general practitioner or his cardiologist (or better, a combination thereof). Most people who have valve surgery experience a great lessening of symptoms and heart issues that were related to the damaged valve, which comes as a boost to general health. So it's likely he'll be able to go back to using it, or he might even find that he doesn't need it as often.
If they cracked his rib cage (sternotomy) to do valve surgery, he'll likely not be in typical missionary position for another month. But the equipment itself is traditionally unaffected by the surgery.
You can try Al Lodwick's warfarin site to see if there are caveats for Viagra when on warfarin, but I'm sure we'd all have heard about it by now if there were. It would definitely have been a bone of contention.
Best wishes,