Climbing systems are built on redundancy, and you should always consider establishing a backup for a lower. There are situations (rain, darkness, icy rope, et cetera) in which a backup will greatly enhance security. The simplest way to back up a lower is an extra hand. When available, have a third person feed the brake side to the belayer during the lower. If the belayer loses control, the backup belayer can hold the weight of the climber and even take over the lower.
If no backup belayer is available and the lower is being done off the anchor, a simple backup can be established by attaching a prusik or similar clamping hitch to the brake strand of the rope and clipping it to the belayer’s harness.
The belayer’s free hand can keep the hitch loose during the lower, but if anything goes wrong, it will hold the weight. (If the belayer is lowering from a device attached to his or her harness, the clamping hitch should be attached to the loaded strand instead of the brake strand and kept loose with the guide hand.)