I researched this a bit more and I uncovered that this is an open question. There is no clear consensus about whether antioxidants are beneficial or harmful to patients undergoing chemotherapy. See:The Good, Bad, and Uncertain: Antioxidant and Chemotherapy Combinations
For a source saying that this is still an open question. If you want a source of what looks like a fairly reputable survey saying that antioxidants actually increased survival, look here
I'm not a medical professional, but my intuition is that antioxidants here are probably helpful, not harmful. And personally, if in that circumstance, and given that advice by a doctor, I'd ignore it and keep going with whatever tea I was drinking. But I'd focus on natural antioxidants, such as naturally-occurring vitamins A, C, and E, from whole plant-based sources, rather than relying on the Catechins or Theaflavins/Thearubigins in tea (which are not essential nutrients). I hope this helps clarify!