I would try talking to the network admin on a one to one basis. Be understanding (remember they are a junior network admin). This is where you get to interview them. Make a list of things you want/need to have installed/configured. If she is still unwilling to turn over the reigns then what you have to go is play along. Stay very friendly at all costs. Ask for changes on an ongoing and nearly continual basis. Do everything you can to make sure the technical ability of this person will be far surpassed. Ask for things like mirroring and replication that will be WAY over their head. Find a way to make sure you can cause something to fail that she can't do without help. All kinds of things that require elevated privileges. Make sure that you email these requests and cc the boss so it is clear that you are waiting on something to be done that you could do yourself. It won't take long before she concedes and gives you the access you need to do your job.

Remember above everything else this is a junior person and therefore likely thinks she can wear every hat, even the ones she doesn't understand.