'Just chill with her' is usually said to a male, but can also be said to a girl, in order to advise them to relax or to encourage them to approach a person of interest. The person of interest can either be a person whom you hope for a relationship, friendship, network, or hookup. The most often person to 'just chill with' is Katy Perry. 'Her' is not always a female.
There doesn't even need to be a person to 'just chill with;' a person can 'just chill with' himself. In essence, it means to take it easy.
Bacon: Woooowww whatever just CHILL with her!
2- Bacon: Woooooooooooowwwwww I hate life, I have alllllll this gar homework to do!
Gar: Bro, just chill with her.
3- Gar: Man, I really wanna get that job! The employer i was talkin about, John, is diggin my portfolio.
Bacon: JUST CHILL WITH HER!