Morally yes
Not legally
If both people signed lease, and one walks away, landlord can enforce full amount against remaining tenant*
I would be very skeptical lease was signed specifying each tenant pays 50%
If one person signed lease, only the person who signed is responsible regardless of who walks away (but in this case the non-sign can be forced to vacate)
*yes it maybe possible to pursue civil action, but in this case for such a small amount on someone no longer in same province the costs to file & recover amount would be greater than the amount so no way it ever happens
I'm sure you're correct that the landlord's lease did not make each responsible for 50%. I'm willing to bet that it did the opposite — similar to joint tenancy ownership — making each responsible for ensuring 100% was paid, because each was entitled to full use and enjoyment of 100% of the rented premises. The fact that one of the parties chose to move and not to avail themselves of that use and enjoyment is not the landlord's concern. So as long as the OP continues to act honestly and in good faith, notifying the landlord of his new address and intention to honour the terms he agreed to, there should be no dispute there.
The OP's question was about his co-tenant's rights, if he wants out of his deal with her and the landlord: Specifically if he cannot get them to willingly agree to what only he wants. I'd say his only option is to run, covering his tracks as best he can and hoping neither has the money to track him down and make him pay what he owes. While he would have a certain power to force his landlord to accept a sub-lessee if he was the only tenant, he has no such power to force his co-tenant to accept anyone as her roomie, and he must have her agreement before the question even arises of what the landlord thinks. If he'd merely rented a room from her, he could argue she was his landlord and had to accept his sub-let, but he didn't.
He either keeps his word now, welches and runs or commits to the grown-up way of dealing with changed circumstances: Negotiate, persuading everyone to agree even if no one gets everything they want. Offer some of the money he's going west to earn to buy his way out of the three-way deal he agreed to. His roomie might be willing to pay the landlord a bit more, as long as it wasn't the full amount, for the bonus of having the place to herself. It's in no one's interest to pursue him in Small Claims, and even worse for both of them if her lack of funds makes her do a flit. But he'll have to do some smooth talking to persuade them to take a hit at all.
No one twisted his arm to sign that lease.