It is difficult to second-guess why a person might make such a request. However, If you are at all uncomfortable with it, then do not comply. There are thousands of photos of stomas on the internet (and many on this site) that someone could look up if they were at all interested in the subject. Your body is your business and, this someone would need to have (and explain) their reasons for such a request in detail, to be convincing that it was not for some malevolent reason or simply for their own gratification. I would tend to consider this request with great caution and tentatively categorise it as a diguised attempt at a wierd perverted porn request, which, at some time in the future, you might well regret if you complied with the request.
Having spent a lifetime dealing with Machiavellian and malevolent people, this is just my own, somewhat biased take, so I hope others will chip in with opinions on this subject.