This is a true story, but i worked for a woman who had a 4 year old who was still on the boob. It was awful to witness, but he would stamp his feet and start ripping her top open if she didnt feed him. there was nothing worse then watching a 4 year old demand boob
Actually in some countries it is common for 4 year olds to still be having the boob occasionally, and even in Australia there are more mums than you'd think (some who are here on BB I'm sure) who choose to breastfeed for an extended period of time... Just because this isn't your choice doesn't mean it's fair to call it 'awful'.

In response to the OP I would be thinking that there is perhaps something she is still getting out of the bfing... whether it's closeness with mum, or undivided attention or some other emotional need... Perhaps you could try finding out what that need is and seeing if it could be met in other ways more acceptable to you both.