LOL... Like he/she said but perhaps calmer, you still have to give credit but you need to ask if its okay to use her line-art as a reference. If she gives you permission, you will still need to credit her/him (Sorry if i'm gender confused). I hope this helps and please remember even if it sounds like a stupid question, please ask anyways because I would rather you ask then commit art theft which is a BIG NO NO. XD