Muslim Men can't marry Sikhs. But what if that Sikh isn't practicing and thinks of Sikh as just a label can he still marry her because she doesn't really consider it her religion?
بسم الله الرحمن الرحیم
You said ..."what if that Sikh isn't practicing" ... on the whole, it is considered as her religion, therefore She has the ruling of Sikh.
You can compare the example below with your issue (as a helpful example):
For instance, you can consider a Muslim/Muslimah whose religion is Islam, but commonly he/she does not practice it --such as doesn't prayers and fast...--, as a result you cannot say that he/she could not be considered as a Muslim, then he/she has the ruling of a Muslim. Since his/her faith is in Islam.
Accordingly, you cannot consider her as a non-Sikh, since she has chose her religion as Sikh (even she does not practice it).
On the other hand, if we suppose that she indeed doesn't believe in Sikh (and just as unreal label ...), consequently it can be elucidated based on available info. in the answer that she is neither Muslim (nor Ahlul-Kitab). So, according to the rule of ... she as a non-Muslim cannot marry a man Muslim.
http://www.islamquest.net/tr/archive/question/fa1201 (in Turkish)
http://www.islamquest.net/id/archive/question/fa1201 (in Indonesian)