Obviously I'm not a Californian, but there are several things you have suggested that just will not (pardon the expression) fly.
California does not recognize any other state's permit/license. Looking up that information is not difficult.
Borrowing a HG from somebody in Calufornia is going to be problematic at best, especially if she does not have a California CCW, as unarmed open carry is restricted depending on where you are. Looking up that information is not difficult.
California has some wierd (to us in Free America) laws about firearms being registered and people who are not the registered owners possessing those firearms. Looking up that information is not difficult.
Getting a restraining order is a good idea, but needs the time necessary to go through all the steps. It probably also would need the services of an attorney to handle getting it before she arrives in California. That will be difficult as the person she seeks to restrain is entitled to contest her petition and confront her in person in court. Looking up that information is not difficult.
I have no idea if tasers are legal in California. Looking up that information is not difficult. (BTW, tasers, like handguns, are not magic talismans.)
Taking a friend or two to accompany her might be a good idea, as some folks are less prone to violence when there are witnesses present. The question is, will these friends be able to do more than call 9-1-1 if things go south? Also, see below: will taking those friends along involve them as accessories before, during and after the fact to the commission of one or more crimes?
From what you have posted, it sounds as if your girl friend wants to get items that are currently located in the house occupied by her ex. Unless she has a court order giving her possession of specificly listed items that he has so far refused to allow her to take possession of, she needs his permission to enter the place and to remove any property inside. Without those two permissions she will probably be guilty of either trespass or breaking and entering and of some flavor of theft of property, not to mention the possibility of a destruction of property crime or two as well. If such a court order exists and the ex is refusing to comply with it the proper response is to go to the court and ask that the order be enforced. She may or may not need an attorney to do that. A lot would depend on how old the court order was.to make him be out of the house when she comes by to get stuff
Alternatively, she might consider communicating with her ex before she makes the trip (perhaps not even mentioning her travelling to California) and asking him to deliver the items she wants to a third party. It is not unusual for folks to have things shipped across the country. She could then collect her items from the third party. Then her only concern would be managing her visits with family/friends so as to not be near the ex.
Hope these thoughts help you and your girl friend consider how to accomplish her desires without getting into legal trouble.