Ah collies. They are quite impressive dogs even though they have taken a beating with the breeding to make their poor little heads sleek like a racing car.
Boxer? Yeh don't worry about your friend having a lot of intelligence coming out of their dog. They are such sweethearts and fun to be around but they just don't really care about much except food, water, toys, treats and attention. Collies on the other hand are constantly thinking.
Your best bet is to just do a simple lock. Even the little eye locks are great but you have to put it up high enough that your pup isn't able to reach it. But with that kind of intelligence, those little things would be jiggled out in time. So get a different doorknob with a lock on it.
Animals are great. Leopard and I have two cats and they give us our run in laughter. I'd love to have a dog as well but nowhere near enough energy for one unless it were small and Leopard calls small breeds "munchies for real dogs." It's just a joke as he loves all dogs but doesn't want a small one.
I notice Skimble, our eldest, preening in the mirror at times. One night I watched as he puffed himself up and went, "Mrew." He then looked sideways and puffed up a bit different and said, "Meow." He did this several times in different positions with different sounding meows until I couldn't hold in the laughter any more! He was just way too cute!