have you ever considered the lovely city of krakow, poland. i went there earlier this year and was superb. it has everything your mum looks for in a holiday and at the same time there is plenty of other tours different to the bog standard one's and all of them are good value for money. the cost of eating etc... is very reasonable. the only downside i would is the weather, it will be quite cold around december.
one other place i would suggest is koln. it's also reasonable with a great choice of museums etc.. and the food is something else (in a good way). christmas and new year is especialy special in germany. ive worked there in the past for 2 years and last year went for 5 nights to see my old haunts, brought back so many good memories.
if it's somewhere a wee bit warmer you could always go with somewhere like barcelona, malta or lisbon.