What could happen to someone with manic depression keeping it a secret plus leaving it untreated?

Serious. In a worst case scenario, the person could die, either by their own hand or because of extreme/psychotic behaviors while manic. Undiagnosed, unmedicated, and untreated bipolar disorder can tear families apart and destroy marriages.
Serious. There are different types of manic depression, some with more dire outcomes than others. There are two major problems if not addressed. First, the active periods of both mania and of depression adversely affect both career and relationships. These effects accumulate. Second, as one ages the manic and depressive phases "fuse" into a mixed state with a high suicide risk.