What were the flattering words of the spider which made fly get flattered?

The spider compliments the fly's beauty and says that if she steps into his parlor, he will let her view her loveliness in a mirror that he has. At first she says no but flies away calling him "gentle sir," which shows that his flattery about her appearance is beginning to work. Of course, a fly is not generally considered beautiful at all.
When he sees her again, the spider repeats his flattering words about her beauty, saying the following:

Come hither, hither, pretty fly, with the pearl and silver wing:Your robes are green and purple; there’s a crest upon your head;Your eyes are like the diamond bright

Instead of flying away, the fly comes closer, thinking about her beauty. Focused on herself and the idea her enemy has planted in her head about her loveliness, she begins to lose her wise fear of the murderous spider.
As she comes closer, beguiled by his words, the spider strikes and makes a meal of her.
Interestingly, the fly is gendered as "she," perhaps implying that females are prone to believe flattery about their looks and that this can lead them into dangerous situations.