Depends on the circumstances, when the two of them signed a lease, they are -jointly- responsible for making sure the rent is paid. So if she leaves and rent ends up not being paid, then both of them get into trouble (via landlord taking legal action).
Basically the only thing he could probably do is cut his losses if they end up being sued by the landlord, or if he ends up having to make up the difference for the remainder of the lease. The only way I can think of that happening though, is if there was some sort of contract between your friend and the other tenant (not the lease, that involves the landlord, I'm talking something strictly between the two tenants) stating that each would agree to pay an explicitly stated percentage of the rent each month for the period of the lease and having both tenants sign that agreement on paper. If he has that, then he can take that agreement to a small claims court and sue for reparations of the other part of the rent he was forced to pay for the rest of the lease or losses incurred via being sued by the landlord if that should happen.
But if he doesn't have that tenant to tenant agreement, then he's really in kind of a bind, could try finding another roommate to complete the remainder of the lease time with (and get an addendum made to the lease contract), with this option though, the departed friend pretty much gets away with with not having to pay rent for the rest of the lease, but at least your friend doesn't have to shell out any more money than he already is monthly for rent since the new roommate can cover the other part now(ideally).