You'll need the software tools to compile the assembler code (.asm) to machine code (a .hex file) and load it onto the chip via the USBtiny.
Check out ladyada's AVR programming tutorial: http://www.ladyada.net/learn/avr/
That tutorial doesn't explain how to compile the .asm source (and I haven't written assembler code since before the AVR was invented). If you don't have the .hex file, get your friend to explain how to generate it. If you're lucky he set up a makefile to make it easy.