Besides even if Rebecca believed Leon's words and decided to help him, that won't lead to anything.
In the novel, after king Maximus announced the engagement between both of them, he promised her if she needed anything, and he'll do it for her.
After all, he was too happy then. King had just managed to tie the Blake family to himself, and now he had even more power and control over everything.
All this happened because of his daughter, who agreed to this marriage at once, and didn't even question anything.
It was then, that Rebecca put forth her promise that she made with Leon, and how she agreed to help his mother.
King glanced at the people present in the hall, who also heard his words, unable to refuse, he agreed to her request.
Then king asked Baron Darren to take royal healers and they'll go take a look at his wife,
