Yash, to be eligible for NEET, you should have completed your 12th, passed in all subjects and secure a minimum aggregate of 50% in Physics, Chemistry and Biology. It is 50% for General Category, 45% for PWD and 40% for OBC,SC,ST. Now if you belong to OBC Category, the Category you belong to should be also listed in the Central OBC list. Or else you will be considered as General Category and so you have to secure a minimum aggregate of 50% in Physics, Chemistry and Biology. This 50% is inclusive of the Theory and Practical marks scored in Physics, Chemistry and Biology collectively. You needn't score 50% in Physics, 50% in Chemistry and 50% in Biology separately. You need an aggregate of 50% or 40% depending on your Category as an aggregate in Physics, Chemistry and Biology. And along with Physics, Chemistry and Biology, you should also have studied English as one of the core subjects in your 12th. Now final year students can appear for NEET and they should complete their 12th atleast by the time of Counselling and admission. Now NEET 2020 has been scheduled for September 13th and NIOS has postponed its exams but the application filling process has been completed for this timetable. So it is highly unlikely that she would be considered Passed and Completed 12th before NEET 2020 if it is conducted as per the schedule. Because NIOS has not yet released the new schedule. So am afraid she doesnt have the option of NIOS for this year. She might have to appear for NEET 2021 now.

So, as you know you have two options, you may go for any of it. None of the two has any disadvantage except that opting for second option will require one more year while opting for option one does not. Both are valid options. Also, other than 1 extra year requirement there is no harm in choosing the second one. If she is not prepared for NEET completely, then it is advisable to prepare well for it till next year and appear for NEET in 2021.

If she is prepared and can score well in NIOS on demand exams and NEET both, then it is advisable not to invest one more year, register this year itself and qualify the exams through NIOS board and appear for NEET 2020.

Hope it helps.