HR218 gives law enforcement an exemption. Tell her to read up on it to be sure, but it is my understanding that Federal establishments and private businesses have the right to restrict firearms, to include OFF DUTY law enforcement. Now, depending upon how her department defines OFF DUTY, the park may or may not be able to legally restrict her from having her weapon.
For example, I work right outside Washington DC and am friends with a couple DC officers. They are REQUIRED to carry a department authorized weapon when within the city limits. Therefore, they are not being paid, but are considered ON DUTY when within city limits. A lot of federal buildings within DC prohibit weapons. They are either required to provide a secure lockbox for DC plainclothes officers, or they have to grant an exemption to them.
That being said, I carry my firearm concealed just about anywhere I go in the US. If I go to a federal location that specifically prohibits firearms, I find a person in charge and ask for a secure location to lock my gun. More often than not, they tell me to just carry.