Well, if you're 100% sure that you're always with him, you might as well drop Spirit Tap since that talent is beneficial when you solo. It can proc while in a party but the odds are against it so 5 points there is wastage.
So start off with Wand Spec since you will be wanding quite often to finish off mobs when no healing is needed.
Aside from that, pally and holy priest combine pretty well, so picking up the Holy talents that increase damage/crit chance for Smite and Holy Fire are useful.
So to start off with this:
http://realmplayers.com/Talents.aspx#bVZ0xThen go towards the cookie cutter raid/instance healer build, so something like this at lvl 40:
http://realmplayers.com/Talents.aspx#bVMhcZbxtc0bHoly Nova isn't strictly necessary since your pally-friend will have consecrate but if you ever need to farm DM E or run into a big pack of critters, it's useful. Plus you can use rank 1 to flush out rogues. Inspiration is a must to help tanks and Searing Light will increase your dps. Meditation is a must to help with mana. Improved PWF and PWS are quite useful both in PvP and PvE situations.
After that it's really up to you. Since paladins have no HoTs, you might want to pick the Improved Renew or you might want Inner Focus so you can cast Prayer of Healing for free every 5 mins. Improved Healing is a no-brainer if you're going to focus on being a healer. Divine Spirit is another useful buff that many priests pick, so you might want to work on that.