Your buddy has given you the gift of choice, as virtually any class can pair well with a feral druid since they can dps, tank and heal (yes even ferals can heal quite well). So you can choose a DPS, tanking, or healing class and be fine.
Whatever you choose your best bet for fast leveling is spec'ing into the trees that provides maximum DPS, unless you plan on leveling mostly through instances where having a true tank and spec'd healer would make LFG a snap.
So if you're gonna level mostly duo'ing I'd recommend you take the more common "max dps for fast leveling" strategy as feral + shadow. You'll still be fine, as you'll kill most stuff before you NEED healing, and feral druids and shadow priests can both still heal when the need arises.
As far as late-to-end game, while some druids switch to Resto at or after 60, it's not at all automatic or necessary, because feral is 100% viable and valued for end-game tanking and dps. If your buddy says it's likely he'll want to be an end-game healer then you might not want to go the priestly road, unless you want to be an end-game shadow priest. If you've really got your eye on end-game then feral druid + holy priest would be great as you'd be walking around as a tank and healer duo looking for the DPS types which grow on trees.
That's my $.02 anyway.