She's bullsh1tting you. You can't prevent lice with poison.
You couldn't pay me to put insecticide on my kids' heads preventively. In fact, you'd be hard-pressed to get me to do it even with an active infestation. I'm just not into poisons. Henna is effective against lice. Henna combined with fenugreek or artemisia is supposed to work even better. I use henna regularly on myself, but I don't want to use my good henna on kids (who don't really want red hair anyway), so I keep some low-dye-content henna and fenugreek in my house, just in case my kids ever get lice.
Mehandi.com shop - Henna will cure head lice, dandruff and ringworm