|Mort the Cleverly Named|
As to whether or not to allow him to bring it along, it is really up to you. Is the group cool with it? Are they prepared to deal with bringing a giant alien monstrosity along with them while trying to gain people's trust? Balance wise it probably won't be too bad, as it will quickly be outclassed by enemies and its size will keep it out of many areas to come.
I should say, however, that it would be useful to review the exact wording of command plants. Specifically, that it perceives your actions in the best possible light, and doesn't attack you. Gobbling up a couple of caravan drivers would be completely in the creatures nature, and an opposed Charisma check would be required to stop it (tough for many Druids, given that Charisma is often a dump stat and the Moonflower has a 17). Basically, remember that this isn't dominate monster. It makes the Moonflower a friend of the Druid's, but is still an abomination from beyond the stars, and should be treated accordingly.
It has cold resistance 10, pulses with light, and has an environment of "any land", so it'll probably be fine crossing the pole.
Here's the real problem:
Pod Spawn (Ex) Should a moonflower's pod prison kill and digest a Small or larger creature, the pod transforms into an adult moonflower with full hit points after 1d4 hours. The newly formed moonflower has its own consciousness, but some aspect of its trunk or blossoms resembles the creature that died within. The dead creature's equipment remains inside the new moonflower and can be retrieved by killing it.
It would a great pet for most of Book Three, but bringing it into civilization would be insane. It would literally be more destructive than all of the villains in the AP combined. This thing could devour an entire village of peasants and spawn hundreds of copies of itself in the time it takes for the druid sleep and prepare spells. Even if they keep it away from people, it could spawn more of itself by catching wildlife and livestock. The moonflower plague would probably destroy Hongal and become a major threat to its borders forever; there's just not much that can fight an exponentially growing horde of CR 8 monsters. The party needs to either develop perfect control over this thing, or destroy it before it grows out of control.