The spin-forbidden dipole-allowed radiative decay process BH(a(3) Pi) --> BH(X(1) Sigma(+)) is studied using second-order configuration interaction wave functions and the full Breit-Pauli spin-orbit Hamiltonian. The radiative lifetime of the (a(3) Pi, v, N, F-i, elf) --> X(1) Sigma(+) transition was determined. The average radiative lifetime was found to be approximately 20 s for v = 0-3 with the shortest lifetime being longer than 10 s.