What is the ruling on visiting a sick person of the opposite gender who is not a relative by blood?

There is no impediment to that, provided the Sharia conditions are met: absence of Khalwa (private seclusion), adherence to Hijab, lowering the gaze, and not speaking in a soft or alluring manner. The prevailing customs must also be considered, as such an act might be deemed disgraceful and dishonorable in some customs.