I just installed a lock in my door and when attempting to install the latch I found that the wood was splintering and not stable, so I cut it out. My problem is similar to this one. What can be installed in the gap to make the latch have something strong to screw into? Best type of wood to use? Other ideas besides wood? Replacing the entire door frame is not an option at this point. Reference image:
I suggest replacing that jamb board. Measure the board and purchase a new board to replace it. Cut it completely out, top to bottom and install the new board. Cut your lockset holes and paint to match.