both answers are great! depending on the client and the age i prefer doing back bridging. Start of simple and progress by placing both legs to one another, adding weight or even one legged back bridging to show progression. Important thing is to go slow, making sure the client feels the pull in every motion.
Once the client becomes accustomed, i would strengthen the hamstrings. The reason being is due to the muscles attached from the erectors to the hamstrings. When someone gets LBP, they compensate on hamstrings for the lift, so strenghtening them would help mucho!
Weak back, can also be from a strong core. Always train the antagonist muscles, never overtrain a body region without doing the opposite.