Where is the guy who needed coffee at pewter city on pokemon fire red?

You're likely thinking of the Coffee Addict in Pewter City. He's located in the southwestern house of Pewter City, just outside of the Gym. He's the man sitting on a chair, and he'll ask you to bring him some "Strong Coffee".

To get the Strong Coffee, you'll need to head to the Celadon Department Store on the 7th floor. There you can buy Strong Coffee for 500 Pokédollars.

After delivering the coffee, he'll be very grateful and give you TM11 (Sunny Day) as a reward!