Occasionally stuff like this happens in the wild. Those of us who have not had the opportunity to experience a true wilderness find this hard to believe. It is uncommon, but not unheard of. They are not going to be 'friends' forever. Eventually, the predator will eat it or leave it or the prey will take off.
The lion looks like it has some spots; is that an indicator of a juvenile lion? If so, that could explain a lot in this particular situation. Or the lion could have brought the prey for her young. Or the lion is not hungry and is playing with it. Lots of reasons.
Why do most assume this is a lioness? Why can't it be a lion? Because it doesn't have a mane and we anthropomorphize that this lion is nurturing the antelope, we assume it must be female. Any juvenile lion could display this behavior.