Legal Question in Landlord & Tenant Law in Illinois

The question really is, to whom is she liable?

1. Assuming a "typical" lease and the way they signed it, each of them probably (an attorney should review the lease for certainty) is "jointly and severally" liable to the landlord for the rent. That means the landlord won't care who pays as long as the landlord gets the rent. The problem occurs if the boyfriend doesn't, but unless he wants to run the risk of an eviction, he may just decide to pay the full boat.

2. As between your daughter and the abusive boyfriend, they had what one might consider to be an "oral general partnership" evidenced by both their names on the lease, evidencing that they both knew or should have known that while either could be fully liable to the landlord, as between them there is some evidence that each was to share 50/50 in the rent.

3. The problem comes down to this if he sues her: if she took no action at the time of the abuse (police report....) and it may be too late now especially if he is leaving her alone otherwise, how can she prove that she was forced out by his behavior, which might be a breach of their "partnership" agreement or some kind of "constructive eviction" (although that theory usually is applicable between a landlord and tenant and not between tenants). If it is only his word against hers, there is a good chance that a judge will not be able to give her relief. So yes she could easily be held liable, but one way to try to forestall a lawsuit may be for an attorney to write the boyfriend that regardless of the nature of the case the defense would be his abusive behavior; and if he then decided to sue her this would come out. It might just deter him but there's no guarantee. Alternatively she should consult with a lawyer and if he is continuing to make not only threats of suit but she is in fear that he may yet physically harm her, there may be current grounds for a civil restraining order. This too would require proof.