£30-40 plus postage seems to be the going rate for consoles on here. Obviously more if there are controllers and games. Having said that though, there are so many Wii's in the used market these days it may be difficult to get it sold.
Even on eBay they're going for around the same price or less, then fees have to be paid out of that as well.
I won one on ebay with 2 controllers for £10 but the seller is refusing to let me have it because it didn't go for as much as they wanted. I'd stick it on ebay if you just want it gone, if it has rare games like certain Mario ones then people might bid high.
Sometimes I think there comes a point at where something isn't worth selling. It happened with the Dreamcast and Original Xbox and now with the Wii. Tell her to put it in the loft and dig it out again years later for some nostalgic fun