Possible Theory
Can it be posited that Laura Manion was flirting with Barney Quill while playing pinball, gave him her panties in an act of flirtation, and then told her husband she was raped?
Think of the following:
1) Ms. Manion tries to give Mr. Biegler her girdle towards the end of the film; this would be similar to her giving Quill her panties.
2) It was stated, when Biegler and Manion are in the car, that she is in an unpleasant marriage - "but I can't leave him." The implication, here, is that Frederick Manion would be furious at such an attempt.
3) As a way to get her husband locked up for good, but also maintaining her 'devoted wife' role to Lt. Manion, she, in a round about way, gets Manion to kill another man. She thinks for sure he will be in prison indefinitely, and she will be freed from the marriage.