Srilanka can buys its weapon from anyone it wishes and India Or any Country doesn't have the rights to ask Srilanka to buy weapons from them.
But What India can certainly do is Market thier weapons to attract countries like SriLanka.
I think Our National security advisor's concern would have arised because of fear that SriLanka would use those weapons to attcak India which is understandable. But the approach he took to go to the media to say such a comment is deplorable. Instead he should have had a meeting with the SriLankan officials to market India's weapon and see where we lack and develop weapons Or Market them to attract them.