Medically the Oncologist will make an assessment based on your mother's specific case. The staging, the location, the past history etc. There is also the logistics of transporting her every day, which needs to be taken into account. She will have good days and bad days, but still have to turn up on bad days.

I suspect the Oncologist thinks the daily treatment option is medically better, with a better prognosis. Otherwise he would just recommend the easier weekly travel option. Ask him if thats the case, he'll have an opinion whether the better prognosis is worth the extra stress.