I would be very annoyed if someone brought their child to my house knowing they had nits. (Addressing the OP's query here, not your post Netix - I read your post to indicate that your friend didn't realise her child had nits.) Having said that, you have five days to treat the nits and so you should still be fine to go away. I would be treating again and making sure to use a nit comb and a load of conditioner each day until you go away to comb out any eggs. You can buy the metal lice combs from the chemist for a couple of bucks. There is a product (Moove??) in a green box - it's a conditioner with natural ingredients to kill lice and comes with a metal nit comb. You can get shampoo as well in the same range. My DD1 has hair down to her bottom so although she had only had nits once in her life (when she was about 2 and had very little hair!) I comb the conditioner through her hair every month or so just to be sure. Make sure you also wash all your bed linen, towels etc in hot water and maybe throw a cap full of ti tree oil in the wash as well.

Like I said, I would be really annoyed if someone knowingly brought nits into my house, you've seen how hard they are to get rid of!