I'm helping a friend with his 71 Challenger issue:

Started out with low beam headlights went out - car already had new dimmer switch, and all other lights work; dash, tail, parking.
But when the dimmer switch is activated on high beam, all 4 lights work.

Thought maybe it was the headlight switch, so he bought a new dash switch and I swapped it out for him. The low beams then worked.

Fast forward to last night he was at a cruise night and when he turned on the headlight switch, the same issue, no low beams, so he had to drive home with all 4 lights on high beam.

Before I trek out to his place, looking for suggestions?

Could the low beams on both side be bad? If so, why would they go on when the high beam is activated?
Could the new dimmer switch be bad?

TIA!