The more I think about this, the more difficult it becomes! Here are my two cents: You would be understood if you said "vennlig med henne", but it does sound a bit strange to me. If you want to describe a way of acting directly towards a person, then "mot" would probably be the better option. "Jeg er snill/slem/likegyldig mot henne" (I'm kind/mean/indifferent towards her). "Hvordan oppfører du deg mot han?" (How are you behaving towards him?) If you use "med" instead in this sentence, the meaning changes slightly. "Hvordan oppfører du deg (når du er) med han?" (How are you behaving when (you are) with him?)
However, you could use "med" when reflecting over how you're handling/treating her. "Jeg er forsiktig med henne" (I'm careful with her). This could also mean that you're acting carefully when you're around her, not just towards her. "Med" can also be used when ordering others to behave a certain way towards someone. "Vær snill med henne!" (Be kind to her!), here "mot" would also work: "Vær snill mot henne!" (Be kind towards her!)
Moral of the story: "Mot" is the safer bet when describing how you're treating someone. "Med" can also be used in some situations, but be aware that this could change the meaning slightly to be more about how you're acting yourself - not necessarily towards someone.