Some Questions about Probiotics?

My Mom was just recently diagnosed with TNBC, she is stage 1 with no lymph node involvement and just had a lumpectomy.   She will be starting Chemotherapy in about 2 weeks and of course I've been researching everything I can to hopefully help her get through it as best she can.   Her Oncologist say's that the chemo side effects will probably affect her more than others because she is significantly overweight and chemo is based off your weight/height.  One of the thing's I've read is that supplementing with a good probiotic can significantly help with the naussea/diarrhea associated with chemo because it helps to replinish the good gut bacteria that get's wiped out by the chemo.   Problem is that there are about a million different kinds of probiotics on the market and I want to make sure I'm getting a good one that will actually help.  Can anyone offer some assistance with choosing one?   And also, is this something that she should take on a daily basis or just after the chemo?