Note that you can wire these in parallel to get 4 ohms, but they'll always sound too bright because the tweeter will be drawing as much power as the mids.
Not if he's using a crossover. If the tweeters are on high pass and the mids are low pass, then the amp will see the load as one speaker, a final 8 ohm load. You'll need an amp that puts out twice as much as the required RMS for those speakers at 4 ohms (if your speakers are rated for 100W RMS, you'll need an amp that's 200W @ 4 ohm).