You missed Sonja (1970)

Rick Hardin, a mid-30s author of pulp novels, just wants to pick up his girl Sonja at a lonely gas station. But it seems that the cab has already picked her up. At the rest rooms, he witnesses a man battering a woman. Is it Sonja? Rick decides to help, but has to realize, that the victim is not Sonja at all, but a weird woman, who does not want to be helped. And he has to admit, that life sometimes holds strange twists. Unfortunately, this realization seems to come too late.