Unfortunately, if her command will not give her a weekend pass or leave, there is nothing that she can do to legally force them to let her go.
In the military, if you are not on a pass or leave, command can control where you go and what you do. It is a part of military service that can be very unforgiving. Leaving without permission amounts to a criminal act in the military, so I would make sure that everyone is counseling her to only leave with permission. While it is completely understandable for her to want to get married and to follow through with her plans, she's going to need people counseling her to follow command instructions. I'm married. I know how important that day is, but a permanent federal conviction would forever alter her future options for jobs, school, etc.
Now that I've said all that, because I really want to protect her future here, I will give some options to try and get the pass granted. These are not guaranteed. There are no guaranteed options. These are just things she can try.
She can go to the base chaplain and explain her issue. Chaplains have the ability to go in as a neutral third party and work with command to help them understand why letting her go would be a good thing. Many commanders will take a position when they don't have to explain it to anyone, but explaining an inflexible position to a chaplain is a little more difficult. The commander can always retain that position, but making an appeal to a chaplain is a good method.
She can go over her commander's head to the next level commander. This is a little more tricky, because it can be seen as disrespectful by the lower level commander; however, if she is already leaving to a new location, this wouldn't hurt too much.
She can go to medical and express the significant stress that this is placing on her. In some instances, medical can influence the decision that a commander makes.
I wish her the best of luck, but regrettably I have seen many instances where a commander makes a decision like this (when they didn't have to) and there isn't anything that can be done. Military life often results in situations where what we want to do, no matter how appropriate, is not permitted and we have to adjust.