1) We don't know that the fleas are from the lawn. Unless it's been warm there for a week or more, I would be very surprised if the fleas were from the lawn. I'd suspect another animal or maybe a garage-kept car that is used to transport infected animals.
2) It's probably illegal. E.g. [link|http://www.dnr.mo.gov/env/wpp/permits/issued/R401000.pdf|6 page .pdf].
3) It's dangerous.
If she wants to treat her lawn to control fleas, she should use something like [link|http://www.scotts.com/index.cfm/event/ProductGuide.product/documentType/product/category/%2FCategories%2FProducts%2FWeed+Insect+and+Disease+Control%2FLawn+Insect+Control+with+Fertilizer/documentId/4ee0cc56d9f04c3605fb1449d4a69c8c|this]. Note that it's used in the summer. More information about indoor and outdoor flea control is [link|http://insects.tamu.edu/extension/bulletins/uc/uc-034.html|here].
My $0.02.
Cheers,
Scott.