So sorry about your friend. I would dress according to where the celebration of life is being held. If it's in a church, for example, then I wouldn't wear jeans unless I was positive that everyone in that congregation wears them. If it's at their home, you have more flexibility. However, I would think black pants with a colorful sweater or scarf or some big colorful jewelry would cover you. They just don't want all black.
I've never heard of bringing money or checks. I agree with a previous post that it depends on the family situation as well as the regional, religious, and cultural traditions. You don't want to insult them by assuming they have invited people just to collect cash, but you don't want to leave them strapped if that is their situation.
Flowers can be wasteful in some people's minds, and they remind people of funerals, which this family doesn't want. Also, it can be difficult when all the guests leave and the flowers start to die - reminds the family of death. You might consider (by yourself or with a group of high school friends) planting a tree or installing a park bench (both with explanatory plaques) in the town - in front of the high school or the library or a park/nature area, or anything else that relates to your friends interests. People like that a tree lives on and on, and they like that a bench is useful for years to come. Others establish a scholarship at the high school in the friend's name. It can be general, or in a specific area that interested your friend (athletics, music, etc.).
Finally, I know a woman who has a shop that sells stained glass lamps in so many price ranges. She writes a short poem based on info you give her, and frames it. It can be displayed next to the lamp. The idea is that every time the family lights the lamp, the light of the person's spirit fills the room. They are often displayed at funeral homes instead of flowers. She ships anywhere. I think this might be a franchise and perhaps you can find one near you. If not, and if this interests you, I can give you the info. I've been in her shop and have read many of the poems she's written - they are brief and not sad. It strikes me as a nice alternative to typical funeral gestures, and it lasts.
Good luck with whatever you decide.