I think that charlotte is a true guardian of Wilbur because she protects him from threats and other animals that wan to harm him in any way they can. She is also a guardian because she educates him and credits him if he tries something and doesn't succeed.
When the lamb over hears Wilbur talking about how his recent meals were wonderful, the sheep tells him that its because Mr Zuckerman is fattening him to later on eat him. This sends Wilbur in to panic and tears. Charlotte who feels pity for Wilbur yells at the sheep to silence him and tries to comfort Wilbur by telling him its not true. Not only that, but she promises to make a plan to save him. This is an example of how Charlotte protects Wilbur from threats and other animals.
When Wilbur, for fun, attempts several times to make a spider web but fails, Charlotte, to try to make Wilbur feel better, explains to him that he lacks spinnerets, which is a substance that only spiders have to help them make webs. This is one example of how Charlotte credits Wilbur at something he tried to do, even when he didn't succeed.
Charlotte would always tell Wilbur the reasonings of her actions and how she does things. For example when Wilbur sees her trap a bug, Wilbur asks her why she does that? Charlotte explians to him that, she traps bugs for their blood, so she can live. She also explains to Wilbur that before she kills the bugs, she injects poison within them so they wont feel any pain. Charlotte points out the parts of a spiders body too, for example, the Coxa, Trochanter, Felmur, Patella, Tibia, Metatarsus and the Tarsus. This not only amuses Wilbur but educates him as well.
In my opinion, Charlotte is a perfect guardian simply because she does everything a guardian needs to do, educate, Protect and credit to build up someones confidence.