I am not absolutely sure, but HBF is a complex moiety representing an extremely strong acid, K dissoc. of HBF4 higher than K dissoc. of HF.
If it is so, the H2O solution of 4-nitrobenzene-diazonium bor-tetraflorate /O2N-benzene-N2+/./-BF4/ should be relatively stabile. On contrary to other nitrobenzene diazonium salts this one does not decompose easily at moderately increased temperature on N2, nitrophenol and acid.