Because according to the quote you cited, Dandeer was severely injured? Even if she made up the "Jaffur attacked me" story entirely,
someone put her in critical condition.
She may have been in no shape to cast a spell as complex and sturdy as the Sealing would need to be, even if she was capable of lesser magic like putting someone to sleep or totally erasing their short-term memory.
Basically, if Dandeer's motives
weren't more or less as advertised, it wouldn't make a lot of sense for her
not to take the chance to seal Jaffur in his sleep. Even if she needed to appeal to Berra first for political support, as
@Bakkasama points out, she could have done that
without being badly injured. She could have at least tried it.