Lenya was actually thinking about going after them herself. Then she could have just flew 60 ft. above them after catching up (with the Haste still running) and blast at them unhindered. Hopefully being able to force them to leave Kerwyn behind by safely putting him down on the ground then retreat unchallenged.
But I realized, that this is futile unfortunately. They can just double move and it will be impossible to catch them, she would need to be twice as fast to keep up and shoot them. :\
The only thing, she could try is to fly after them with the speedup from the Haste and then bluff them, that she is faster and more agile and has ranged attacks and thus they stand no chance, in order to trade Kerwyn's life for theirs. They can even keep the dagger.
Anyways, if that seems like a reasonable line of thinking (it certainly requires to be able to adjudicate the gargoyle's flight capabilities, which she maybe cannot do, and knowing enough about Fly and Haste, which is at least possible with Spellcraft, and without that knowledge it seems a bit too reckless a plan, I guess ), then Lenya would try it (the last paragraph) hoping to save Kerwyn from certain doom.
Yeah, I will just have to leave that decision to Endur, whether Lenya can adjudicate in the short time, that the gargoyles are not the most agile flyers, because that is really necessary, and that she would have that clear advantage over them with the spells for a short time at least.
At least the bluff is worth a try, and it's not even so unreasonable a bluff following the speedy pursuit and then simply staying out of reach above them, where she doesn't need to go faster anymore.
Lenya's spellcraft and understanding of Fly and Haste is sufficient to allow her to realize that the combination will make her a faster flyer than many natural flyers. She does not know whether she is faster than the Gargoyles.