Would it be regarded as a bad choice Karmically, if someone opted for natural therapies rather than conventional medicine, if they had a serious illness?

You cannot look at this question in isolation, you have to see the rest of their life up to that point.

If the individual has chosen to not go the conventional route it is more than likely they will always have chosen natural therapies when they were ill, for any reason. That is their make-up, and lifestyle choices reflect that.