On 10 man it is for sure. I honestly never interrupt and just have a focus macro set that I hit when I get in there and just start spamming away. The very first time I wasn't sure exactly what to interrupt and just started healing him and that's all I have done since. I just hot up and Nourish spam.
the hostile npc doesnt always cast his channeled spell at same time from my experience. Sometimes I heal friendly guy without ever seeing a spell cast, and other times the hostile guy starts channeling like within 10 seconds of being in the room. So, yes it is possible to do without interuptting if u heal a lot or if the hostile npc doesnt cast it soon. If he does cast it interupting it will be more beneficial than trying to heal through it though.
and just to make sure everyone knows ill say this: the friendly npc does more dmg as he has more health so heal him to full asap. Don't keep him around 50% and try to save mana and then suddenly die and wipe the raid when u havent killed the hostile npc in time.