Has anything changed with your girlfriend, whether it be the soap she uses, having/interacting with other animals (dogs, cats, other rabbits….), and/or is she entering a new space she hasn’t in the past with your rabbit (ex. you moved your rabbit to a new spot in the house and she’s entering the new territory)? Has he always been aggressive to her, or is this a definite new behavior. If new, for how long?
Post-neuter/-spay rabbits sometimes are “reset”, in terms of needing to start over in developing a relationship. Hormones really change things! I would start at the basics with bonding your rabbit to your girlfriend because, regardless of the specific reason, he does not see her as someone to interact with gently. There are many different approaches and considerations to look over here: https://wabbitwiki.com/wiki/Behavior_FAQ
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.