A profitable farm makes its bread and butter on 1) Boarding then 2) Leasing, then 3) Lessons. If lessons are their priority, they will never make money.
I haven't run a barn, but have known a lot of barn owners/trainers, and from my conversations with them, I would prepend this list with 1) Training board. (Which is basically #1 and #3 combined, really.) Most places I've boarded at (hunter/jumper/eventing barns, FWIW) have said that boarding breaks even, once they factor in the cost of labor and maintaining the facilities. If you tack on $X/mo for training, though (which is essentially "free" to provide as it only requires time, provided you have the in-house experience to provide training), then the profit starts adding up.
Is anyone at your fiancee's barn (including her) able to provide training (for horses)? If so, that could fill in her "empty" hours, and possibly also allow you to share a car (if she goes in at 7 when you do, and then you pick her up in the evening when she's done).