Happened to me, I chose a colour near as possible to my own, called latte but it went nearly mahogany instead of light brown. It may be better to bite the bullet and get a professional to do it.
I'd have thought anything worth a try if recommended by a professional. Unless there was a fear of making things worse. Otherwise can only let the experts sort it and consider it a lesson learnt.
Thanks for replies,she ended up going to hairdressers and it was done a light brown,looks very nice as some of the light colour shows through a little ,a bit like highlights.She swears she will never do her own hair again...but leave it to the professionals!!!!