I don't doubt that her fate was sealed as soon as the suspect saw her but what reason would the suspect have to be in the park all night? I mean what if he waited there all night and Thelma didn't show up there on her walk? He could have been waiting a while for his victim.
It's not unusual for some people to drink in a park throughout the night. Crime of opportunity is a crime that happens because an opportunity presents itself. Thelma was at the wrong place at the wrong time, and she met the suspect. She was attacked, she lost her glasses and cell phone in the struggle, and she was taken out of the city. If she had not been there alone and vulnerable, maybe it would have been a different victim, and maybe there would have been no victim.