Having an ostomy can make things like this difficult. While I'm sure it's "possible" to surf with an ostomy, it is going to make things much less comfortable.
That said, if he is sick, he isn't going to be doing/enjoying surfing much anyways. At least with the ostomy he will feel better and be able to pursue an overall more active lifestyle, even if it doesn't include surfing. Unfortunately, sometimes we are forced to pick the better of two less-than-ideal options.