My roommate left and stopped paying rent without notice because she thought my boyfriend was living with us (in a 1/1)... Is she responsible for paying her half of the rent after she left?

Probably, depending on what state this is and what written agreement you had, with each other, and or the landlord.
But getting paid is another story, even if Small Claims rules for you or the landlord, whoever sues her. Your landlord may decide it's easier to sue you, or both of you together, depending on how the lease is written.
A judge might even believe that she had good reason to leave and give her some leeway. Roommate cases are rarely cut and dry, legally.