According to your description (your sexual partner's back is affected with herpes), is it possible that your sexual partner has what is called a "herpes zoster lesion". Herpes zoster is a recidiv (reoccurring) infection when the primary infection is caused by the varicella herpes-zoster virus. During a primary infection (usually in childhood), the varicella-zoster causes chickenpox (varicella). After tit runs its course, the virus can remain in the body in an inactive state, hidden in the ganglia of the peripheral sensory nerves. In conditions when immunity drops (stress, other diseases), the hidden virus can become active and cause very painful skin lesions. This condition is called herpes zoster. Neither varicella (chickenpox) nor herpes zoster are sexually transmitted.
There is another herpes-virus named as Herpes simplex virus (HSV) that is transmitted by direct contact (including sexual) with the affected skin or mucosal area.