Ash Tyler was supposed to be
dead as far as the Klingons knew.
In season 2 finale, Ash was on the same Klingon battleship with L'Rell when he brought them to help Discovery
This would reveal L'Rell's lies and compromise her position as
head of Klingon Empire
safety of their son.
Why did L'Rell agree to help discovery when there was so much at stake?