My friend has got all India rank 6.7k in jee mains.which is the best institute she can get with this rank in cs branch??(general category)

Hey,
Congrats for such a wonderful score.
You have a good chance of getting into good NIT. You can check for previous year cutoffs of NIT's on the following link

https://www.google.com/amp/s/engineering.careers360.com/articles/nits-cutoff-for-top-btech-branches

For other available colleges with your rank you can check on the college predictor link provided below

https://engineering.careers360.com/jee-main-college-predictor

Hope this solves your query.
Good Luck!!