Of course they are. They can do what they want.
Despite criticisms to the contrary, there is no injunction within the New Kadampa Tradition (NKT) — in the Internal Rules, the NKT books, by word of mouth, or elsewhere — against students reading non-NKT books. Geshe Kelsang’s commentaries to Je Tsongkhapa’s teachings form the basis of the three NKT study programs (the General Program, Foundation Program and Teacher Training Program, see Internal Rule 15); but outside of their studies NKT students can and do read whatever they wish. Everyone has freedom to do as they choose.
One statement has been taken out of context and exaggerated on various chat groups and forums. Thirty-five years ago an NKT Resident Teacher taught his students in a class: “You should not read books other than Geshe Kelsang’s books.” This teacher’s motivation was to help the students to pass their examination by preventing distractions arising from reading other books. However, this has never been an NKT rule or even suggestion.