If the cleric has 18 AC from armor, he's still going to take 7 damage from the wall of thorns. If he's already buffed with Shield of Faith and managed to DMM persist both that and Sanctuary, then he has 20 AC and only takes 5 damage...but he's also used up both of his 1st level spells, which means all he has is melee, 0-level spells, and a single domain spell. The druid has a good will save, and thus has a decent chance of being able to attack the cleric with her sling, too. If she succeeds on the will save, the druid can plink at the cleric with roughly 100 sling attacks while wall of thorns wears off - if we assume the cleric has 18 Wis, that's a DC 15 will save. The druid certainly has at least 16 wisdom, and a good will save. In addition, since you only get one chance to save vs. sanctuary, the druid will cast Resistance and share that with her animal companion. Druid now has a 55% chance to save, while her riding dog has a 30% chance to save. If the druid had 18 Wisdom, she'd have a 60% chance to save.
Could the cleric win at that point? Perhaps, but the druid certainly has a major advantage. She has more than a 50% chance to be able to attack the cleric directly with her sling while the cleric is waiting out the wall of thorns. It's probably reasonable to assume the druid's animal companion wasn't spotted earlier what with the +10 hide bonus from camouflage, so the druid has her companion while the cleric is alone, and the companion has a 30% chance of being able to attack. If the druid was able to wait until just the cleric remained alive, then she's got 4 uses of summon nature's ally to use. If she wasn't able to wait that long, she probably had to use one to finish off the other remaining combatant, and one on the cleric. That means she can cast it two more times. Generally speaking, the cleric is probably going to take at least 10, if not 15 or 20 damage from the repeated walls of thorns, and his healing is going to be limited because he had to use his spells to get his sanctuary up and his AC to 20.
For the cleric, the best-case scenario is that the druid fails her save against the sanctuary AND her companion has already been killed. But the cleric comes out of the wall of thorns anywhere from moderately to severely wounded. As soon as she attacks the druid, sanctuary goes away and the druid can fight back. At that point the cleric is fighting a full-health druid that can keep fighting with no penalty all the way down to -9 HP. The cleric is also wearing medium armor, slowing her movement speed to 20 ft, while the druid didn't have money to spend on armor, so she's got her full 30 ft. movement speed. Perhaps the cleric also has a sling (I doubt he could afford a crossbow if he has all that armor and presumably a melee weapon). The cleric's AC is much higher, giving the cleric a noticeable advantage on that end, but the druid probably only needs to get one to two hits in order to drop the cleric unconscious due to previous damage from the wall of thorns. It's mostly down to luck. If the druid's companion is still alive when it's down to her and the cleric, and the druid is out of spells, the druid almost certainly wins, because once the cleric attacks and thus allows Sanctuary to drop, both the druid and her companion can attack.
So like I said, victory may not be assured, but it certainly seems really, really likely.
Edit: Didn't see the detailed stats before I posted, but I'd still say things are similar. The cleric's probably going to eat at least two wall of thorns spells, so at least 6 damage (assuming he put up shield of faith for one of them). And the druid still has at least a 55% chance to successfully attack the cleric. Alternately, if the druid fails her saving throw, after the first wall of thorns she might wait until the cleric attacks her, thus breaking sanctuary. She then flees from the cleric so she can cast another SNA1, traps the cleric once more, and this time has the freedom to plink away for 10 minutes with her sling.
