I need some assistance on some speakers

It sounds to me like you need some bass blockers for your 3.5's (350-500Hz will do).  Since they're not designed to play mid-bass or lower (like a tweeter) it's quite easy to fry or damage them.  Did you notice a burning smell from the blown 3.5's?  If so, then bberman1 is right, but you should be using some sort of high-pass crossover on your friends' 3.5's.  I've heard of people wrecking top-of-the line MB Quarts because they didn't.  A guy I know even decided to wreck a perfectly good tweeter by putting a low-pass X-over and cranking up the bass boost.  It fried in less than one second. 

If you get a pair that can handle 30 watts rms you should be fine with a X-over.

I agree with the previous post. just find some bass blockers, easily availiable at any respectable shop for under $10.  and like someone said before, tell your friend to turn it down if he hears distortion.  you might also try setting the fader a little bit to the rear so the fronts still play, but not as loud as the much more capable 6x9's. just my 2 cents.

-Will