First, make sure that her ends are not split--that can really cause a lot of tangles at the ends.
Next, make sure she is using shampoo & conditioner, not one of those shampoo/conditioner in one. Pick a shampoo that is for her hair type.
Spray the detangler on and let it sit for a minute or two, then brush.
With brushing, start at the ends and gradually work up to the top of the head. I start at the ends, and brush them all (I throw them all over my shoulder so that it's sort of like a sideways ponytail, hanging down the front of my shirt). Then work in sections gradually moving up. Also, for me, I only brush my hair when it's dry--if I have to do when it's wet, it seems to tangle even more. Do this in front of a big mirror, so that she can see what she's doing, and you do it first, to show her so she can see.
One thing to do is to put on a show and have you gradually do it, going slow so you don't pull at all. Then, once it's combed through and free of tangles, have her try it. Oh, and don't let her use one of those round brushes, where there are bristles on all sides--that will just get caught in her hair.
Personally, I always found that me brushing my hair (once I knew what I was doing) caused less pain than someone else if the someone else wasn't really careful.
Also, you can use the carrot approach--you can offer to do her hair for her in cute braids/pigtails/etc. once she has brushed her hair out.
Another thought is to take her to a friendly hairstylist, and make an appointment, and ask the stylist beforehand to talk to her about caring for her hair.
If this continues though, dad's desire for the long hair may need to go by the wayside--if long hair & brushing is causing lots of misery and tears, short hair and little brushing is a better choice (at least in my opinion :).