She has nothing to fear from Oolanga, who is dead.

Therefore the only reason which could actuate her would be to convince someone else that she was blameless.
This 'someone' could not be you, for you had the evidence of your own eyes.

There was no one else present; therefore it must have been an absent person." "That seems beyond dispute, sir." "There is only one other person whose good opinion she could wish to keep--Edgar Caswall.

He is the only one who fills the bill.

Her lies point to other things besides the death of the African.