It depends on the chemo. Oftentimes, the side effects are immediate, but resolve after a few days. Thus, you may want to visit her a few days or a week after her chemo treatment, so she's not as tired, as fatigue is a common side effect. I had chemo every other week, so I'd have one bad week (chemo week) followed by one good week.
Ask her how she is feeling, and when she'd like you to visit. I liked the distraction of people around me (note: I wasn't sick from chemo, just tired, and had some other odd side effects. Again, the side effects vary greatly by the type of chemo and by the individual patient. I had the rarer side effects listed for my particular treatment, but skipped the more common ones.)