From a tenants perspective, the main problem will often be that to the enforcement of the lease against a cotenant will require a lawsuit. Since lawsuits cost money, the tenant should first consider whether the breaching cotenant would actually be able to pay any award against her. If not, if realistically there is no way to collect from her (in which case, shed be effectively judgment proof), then suing may be throwing bad money after good. An attorney experienced in landlord-tenant law can help the remaining tenant understand her rights, what she could expect to recover, and also the potential cost of taking legal action. Depending upon the dollar amount owed, small claims court may also be an option.
The other big problem is that the landlord has a right to the full rent, even if one of the people paying it has skipped out. As a result, if the remaining tenant does not pay on time, in full, the landlord could evict her. The remaining tenant needs to remain current on the total lease obligations, not just her share, even as shes looking to recover from the breaching cotenant.
As a result, it may make sense for the remaining tenant to consider other options as well as suing the breaching tenant. Subletting, so as to get another roommate to pay the rent is one option. Another option might be to work with the landlord to have the breaching tenant removed from the lease and substitute another person on the lease. Again, a landlord-tenant attorney can help the remaining tenant understand her rights and recourse.