I have observed people in parking lots going to their car in the rain. Normally, they either obstruct their view with an umbrella, or tuck their heads down and move swiftly. People are less likely to be aware of things happening around them (IMO) under these conditions.
...and perhaps may be more likely to accept a ride in the rain. Especially if he had been cyber-stalking her and mentioned a mutual acquaintance to gain her trust. Just a thought.