Mayo really works well, and as stated above, it does a great job at conditioning too.
My other suggestion for the future is to add tea tree oil to her hair care routine. It can help prevent lice. I have found the easiest way is to make a tea tree oil and water solution, just put it in a squirt bottle, give the bottle a shake and spray on hair every morning (or when you add various products/detangle/etc..). The oil is good for your scalp too..