She faints after hearing Willy Wonka's suggestion for fixing Mike's height:
WONKA: So I think we'll put him in my
special taffy-pulling machine. That should do the trick.
MIKE (in the purse): I'm warning you, Mom; there's a nail
file in here . . .
MRS. TEEVEE: Taffy . . .
WONKA: (to an Oompa Loompa) To the taffy-pulling room.
You'll find the boy in his mother's purse. But be extremely
MIKE (in the purse): (on "You'll") If you don't let me out,
I'll [smear your lipstick]*** all over everything.
MRS. TEEVEE: (losing it) T-t-taffy pull-- (as the Oompa
Loompa whispers to Willy Wonka) Oh, what's he saying?
(Mike continues to protest.)***
WONKA: (to the Oompa Loompa) No, no, I won't hold you
(Mrs. Teevee faints backwards into Grandpa Joe's arms.)
She faints after hearing Wonka's response, which suggests that the taffy-pulling might now work - with disastrous consequences, as he says he won't hold the Oompa Loompa responsible if anything goes wrong. This suggestion that something could go wrong frightened her enough that she fainted.
What the Oompa Loompa actually whispered is covered in a different question:
What does this Oompa Loompa whisper to Wonka in the T.V. Room?
It also contains a link to a video showing this scene (unfortunately, a low-quality version):