In Mistralton City (Flying-type Gym city), there is a house within the town with 4 NPCs. Once of these NPCs is the Move Relearner! Once again, this dude wants heart Scales.
Heart Scales can be found on wild Mincinno and Deerling, as well as from another lady in Driftveil City (aka Hodomoe; not recommended for the Japanese version; getting them this way requires careful reading).
Simply have a Heart Scale handy, and you can go to town with giving Musharna that Hypnosis you accidentally deleted!