Hi, sorry thought I had replied to this earlier....but clearly hadn't!
I moved from a vlcd to WW and didn't gain the first week - actually lost 2.5lbs which I was totally amazed at - I STS the second week though.
I think the key is not to go mad with the normal foods and take it easy re carbs re-introduction particularly.
On the first few weeks into WW I didn't eat anything starchy or sugary at all - no carbs other than in fresh fruit and veg. When I did re-introduce them, I made sure it was wholegrain varieties and still try and steer clear of too much bread
The other week I had a mad splurge on carbs and put on 5lbs - did lose it pretty quickly too mind as I think it just bloats you up and puts on water weight.
Tell you mum to keep with healthy foods, porridge, fruit and veg, protein etc and she should be fine - at least for a couple of weeks before going more processed.
I did my vlcd for 10 weeks and by the end of it my body was craving fresh fruit and veg - even though the CD (and the others) is nutritionally balanced, I really missed the freshness of getting my vitamins.
Good luck to your mum and I hope she does well. xx