I would use deadly force if necessary, as Texas law authorizes use of deadly force to prevent the commision of rape of a third person.
Deadly force would not be my first choice. If there was not a weapon evident, I would go hand-to-hand if I think it is possible, or use pepper spray. I do not want to shoot someone unless it is the only option.
If the perpetrator had a weapon or if I did not think it could be stopped non-forcefully without the victim being badly injured, I would draw, and fire if necessary.