i must use Q3 to open the game?
No, depends.
Quake 3 Arena is a game by id software, that is, you must buy it to play it.
OpenArena is an opensource and free alternative to Quake 3 Arena : it works the same, plays the same, but contains different models, maps and gamemodes. But it is completely free, you just have to download it and you're good to go.
To extend the game functionnalities, you can install what we call a "mod". Luckily, mods that are made for Quake 3 Arena are generally compatible with OpenArena.
Now, what you want to play is CPMA, which is a mod. You can install it and play it with Quake 3 Arena or OpenArena. The mod is free to download and play.
But now, if you want to play with your friend, you must use the same game : if he plays OpenArena, you must use OpenArena. If he plays Quake 3 Arena, you must play Quake 3 Arena (and buy it before).
So basically : since your friend plays Quake 3 Arena, you must buy it, and you're at the wrong place. Look at online games reseller like amazon :
Windows version :
http://www.amazon.com/Quake-III-Arena-Pc/dp/B0000296ZDLinux version :
http://www.amazon.com/Quake-3-Arena-Linux-Pc/dp/B000046Q56-------------------------------------
As an alternative, you can either ask your friend to install OpenArena to play with you (but I doubt he will), or, more complicated, try to play with OpenArena assets with ioquake3 binary on Quake 3 servers and CPMA mod (as long as Q3 maps aren't played on server but only CPMA maps, you should be able to freely play on Q3 servers this way, and legally).