Your study partner has concluded that a single action potential, once initiated,

While she is correct about graded potentials, she is incorrect about action potentials. A GP is initiated at a synapse, for example, and spreads in all directions but loses strength as the ions diffuse; no additional ions are crossing the membrane to boost this signal. An AP is initiated at a trigger zone, then a second, identical AP is triggered in the next patch of membrane as more ions enter the cell; thus, there was no decrement of the AP. Between membrane patches, the signal is decremental for the same reasons that GPs decrease with spread, but sufficient to stimulate the next AP. In myelinated axons, the subsequent APs are farther apart than in unmyelinated axons. A row of dominoes, if spaced appropriately, can be felled by pushing on just the first one. That domino falls and does not spread to the end of the row, but it causes its neighboring, identical domino to fall, and so on.