
Spoiler alert
Actually, in the novel, he doesn't believe her. He thinks there's a deeper reason that she's not telling. He starts pursuing her again (like when Kyle wanted to marry).
At the same time, Mathias blackmails Leyla to rape her. Kyle sees them and when confrontring Leyla, she tells Kyle that she loves Mathias and that's when Kyle stops pursuing Leyla and goes to university.
There author wants to "camuflage" the raping as "not knowing that it was love" saying that eventhough Leyla hated Mathias, Kyle felt that the way she said "I love him" was certain

Matthias didn't tape Leyla, he may have forced her to chose between her uncle going to jail or being his mistress, but ultimately she consented to being his mistress to protect her uncle. So even tho we hate his methods he never raped her, she consented to having sex with him even tho she was heartbroken doing it. But knowing what happened in the novel after Uncle found out, he told her that she should have chosen the latter option, he would have preferred to spend a few years in jail if it meant her not being his mistress.

Honey no
This is fiction, so I won't say "let's burn the book and the author"
But that IS rape
Rape means there was no consent
And to have consent :
the person needs to be aware of what she's consenting to (not comatose, not drunk, not too young to understand the act and the consequences, etc)
She needs to be able to take her consent back at any point in time, otherwise it becomes rape
And she needs to be free to make that decision
Otherwise, we would acquit anyone who robs people at gunpoint, because tho they were blackmailed, they readily "consented" and surrendered their possessions
(I said she, but it's not just women who go through that)
I think she's so ridiculous for expecting Kyle to believe that lie.
After all those years together, she still doesn't realize how much he knows her.
If he does believe it, it'll just be bad writing for real