When talking to Mei Wong in Rivet City, you can find out she is an escaped slave and therefore fearful of Sister, a slaver who is hunting a rogue android. While Mei's fears are unfounded and Sister isn't particularly interested to learn she's a former slave, it's still possible to make her overreact and kill herself.
You have a couple of options:
Mesmerize Mei and enslave her again. Rivet City will turn hostile, and you receive bad karma.
Tell Sister that Mei is an escaped slave and let her know you told him. She commits suicide and you receive bad karma.
Provide Mei with 25 caps, allowing her to purchase a pistol to defend herself. You receive +100 Karma and the location of the Temple of the Union.
Kill Sister. Rivet City will turn hostile (including Mei), but you do not lose karma. Coming back after Rivet City calms down earns the ire of the guards in dialogue, but thanks from Mei.
PCPlayStation 3Xbox 360 Mei Wong may not actually buy a pistol after you give her the money. You could rectify this by reverse pickpocketing a pistol and ammunition to her.