The answer was so easy, but it would entail delivering Richard Butler to his death.

Colonel Grant, following his upward glance, beheld Lady O'Moy for the first time.

He bowed, swept off his cocked hat, and "Perhaps her ladyship," he suggested to Sir Terence, "may have seen something." "I have already asked her," replied O'Moy.
And then she herself was feverishly assuring Colonel Grant that she had seen nothing at all, that she had heard a cry and had come out on to the balcony to see what was happening.
"And was Captain Tremayne here when you came out ?" asked O'Moy, the deadly jester.
"Ye-es," she faltered.

"I was only a moment or two before yourself." "You see ?" said Sir Terence heavily to Grant, and Grant, with pursed lips, nodded, his eyes moving from O'Moy to Tremayne.
"But, Sir Terence," cried Tremayne, "I give you my word--I swear to you--that I know absolutely nothing of how Samoval met his death." "What were you doing here ?" O'Moy asked again, and this time the sinister, menacing note of derision vibrated clearly in the question.
Tremayne for the first time in his honest, upright life found himself deliberately choosing between truth and falsehood.

The truth would clear him--since with that truth he would produce witnesses to it, establishing his movements completely.