She faints after hearing Willie 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.) - wonkadotcom
She fainted due to Wonka's response which suggest it might not work as he say he will not hold Oompa Loompa responsible if anything goes wrong which worried her.
What Oompa Loompa whisper is covered in similar question :
What does this Oompa Loompa whisper to Wonka in the T.V. Room?
Which also contains a video showing the same scene but unfortunately in bad quality: