My mate's a pretty decent singer, I think she should go join a singing group or choir or something.
Anyone know of any in Sheffield?

Funnily enough I've been doing some research into choirs in yorkshire for one reason or another and this is one of the best resources for finding them.
http://upperdenby.adsl24.co.uk/MM/mmysocieties.htm

Depending on location, she could try our new community choir starting this Thursday (24th September! Shiregreen Comunity Choir, Shiregreen United Reformed Church, Valentine crescent, S5 ONX at 7.30pm, welcome to start any week. We will be singing mostly popular music and starting some Christmas carols.