Your fiance has great hair! It reminds me of a friend I had in college who was Navajo. She and her brother had this thick, dark shiny hair I would have killed to have myself.
I'd say that the goal of waist length may be possible but don't be disappointed if she doesn't make it. As long as it is as shiny and healthy as it is now, it's going to look beautiful, especially if she's going to be in white. Suggest that she wear it down, at least some of it, and make sure the veil doesn't hide it all, or that she discard the veil for the reception.
Most hair grows only about six inches in a year, so she only has nineteen months to get there. It will only happen if her hair grows more quickly than most. Vitamins will help, especailly A, D and E, and she'll have to have a lot of high-quality protein and vegetable oils in her diet. and if she doesn't make it, don't pout. Quality is more important than quantity, and her hair is beautiful. Please pass this post on to her. And I wish you both all the hapiness in the world!