From a collection standpoint it is going to be tough to collect a dime from the boyfriend. However the lady that signed the lease can and should be held accountable for any damage to your rental. Document everything. Take pictures of any damage and trash. 30 days after move out you may hire a collection agency or report the tenants to Experian, Equifax and TransUnion.
There are several reasons to require all adults living in a rental to sign the lease. This is one. If he had signed the lease he would be held responsible along with any others who signed the lease.
Depending on the extent of the damage and how certain you are that the boyfriend caused it, I would speak with an attorney about other legal options that may be available.
Good luck,
Bill Gray
The Landlord Doctor
http://www.thelandlorddoctor.com Tenant Debt and Tenant Screening Blog
Bill@thelandlorddoctor.com