Your dealer needs to take some electrical classes. Unless there's some magical way the flasher can transform the 12V from the battery into something of a much greater magnitude, there is no way it can hurt the LED lamp. The LED lamps typically operate within a range of 10-30V. As a side note, unlike incandescents LED's dont give one hoot about how many times they are turned off and on.
The current that passes through an LED lamp is determined by the lamp itself and voltage applied to it. If you remove a large incandescent (possibly a headlight) that draws 5 amps and replace it with an LED that is advertised as drawing 590mA, the LED will draw 590mA. What the lamp that was previously there drew means nothing with respect to the current the LED draws. According to his theory, it would mean that if you unplugged a toaster that draws 8 amps and plugged in a night light that draws 0.5A (500mA) the night light would blow up.
Resistors are sometimes added to flasher circuits where LED's replaced incan's because the low current draw of the LED's isn't high enough to make SOME flashers operate. The resistors are put in parallel with the lamp to increase current in the circuit and convince the flasher that there's enough load to operate. EDIT TO ADD: Note that I said the current is increased in the CIRCUIT and not the lamp. In a perfect situation, if the flasher needs to see 1A to function, the resistor would be sized to pass 410mA if the lamp passed 590mA. At 12V that would be roughly a 30 ohm resistor.
Please don't be taking any electrical advice from that guy.