This H M Revenue and Customs Website gives the details your friend is looking for about the childcare vouchers:
http://www.hmrc.gov.uk/leaflets/ir115.pdf
The rate has now gone up and, if their employer is operating the scheme, people can get �55.00 per week or �243.00 per month of vouchers tax and National Insurance free.
Not sure if this is on the website above but HMRC warn that anyone getting, or able to get Tax Credits, will probably be better off doing this than taking childcare vouchers.
If your friend wants to look at more detailed information, try www.hmrc.gov.uk.
Also, if she Googles 'childcare vouchers' she'll be able to look at further information from commercial companies.