You can use the 8 ohm speakers and it won't hurt anything. It will actually be easier for the amp to drive. The down side is they won't be as loud because it will half the power the the amp puts out. An amp rated at 100 watts into 4 ohms will run at around 50 watts into 8 ohms. Your friend is half right. Wiring them together and bridging the amp will make them louder but it won't be less stress on the amp, it will be more.
And without knowing the exact amp you have it probably isn't bridgable. Not all amps are. Mostly its higher end amps geared at running subwoofers in car audio. We are talking motorcycles here, don't they run on 6 volts to begin with? I am assuming its two speakers? If you wire them together and run them to one channel to get a 4 ohm load they won't be stereo anymore, if that matters. If its 4 speakers ( why would you need 4 speakers on a bike?) you could wire them together in pairs and use two channels I guess.
Honeslty I would send them back and get the speakers you ordered or get a helmet with blue tooth or something.