Take a look at the original sentence again:
Running through the parking lot, Aisha hoped to get to her car before the
rain began when she found the doors locked
and saw her keys lying on the front seat, she knew that she was in for a
soaking.
You chose to fix it the correct way:
Running through the parking lot, Aisha hoped to get to her car before the
rain began. When she found the doors locked
and saw her keys lying on the front seat, she knew that she was in for a
soaking.
Aisha hoped to get to her car before the rain began is a complete sentence. When she found the doors locked and saw her keys lying on the front seat, she knew ... begins the next complete sentence. Breaking this fused sentence with a period and a capital W for when would be the perfect way to fix the problem.