From what I have read on this site, it seems that 5FU causes more intense fatigue in the short term than Xeloda/capecitabine (which you could consider it's oral analog). I found that Xeloda fatigued me, but less intensely - more spread out for a week or two - never needed extra sleep and worked after about day 4 (which was, I think, mostly due to the oxaliplatin). I hope your mother manages to cope with this, but if it becomes more difficult, she might ask her onc about using Xeloda instead of the 5fu component of her cocktail".
diag. Stage IV, 5/05, liver met
lap sigmoid colectomy, 6/05
6 cycles Xeloda/oxaliplatin/Avastin (NED after 2)
11/08 9x13mm right lower lobe lung nodule; removed via VATS 4/09
6 cycles Xeloda + Avastin
Avastin only 10/09-5/11
Still NED 06/18