If she really loves flowers then she might also really picky (or knowing
since she's probably gotten a lot of them) about them as well, so if I were you'd I'd make sure they were top-notch! It depends on if she's modern or old-fashioned as well. If she's modern she'd probably like a less "Happy Mother's Day"-esque arrangement and more of a sleek boquet with less filler. Maybe something like orchids, calla lilies, peonies, et cetera.
Or she could just be a flower lover, in which case anything pretty and flowery would work.
I wouldn't suggest getting something in a vase because the vases flowers come in are rarely something someone would want sitting in their house (mine always end up under the kitchen sink--except for one that was rather nice), and she'll probably have a lot of them already since she loves flowers. Plus, it'll give you a chance to see her house and chat while she's putting them in water.