Your friend does not need to worry. Even (real) mathematicians get frustrated reading others' work sometimes, and there are many I know who find Rudin to be difficult to read, especially if it is the first exposure of mathematical textbooks.
This being said, there are several things that need to be addressed that might help your friend.
The first thing is, why is she reading Rudin? Rudin is written with a special focus on proofs. However, in many fields, there are other impeding matters other than proofs. For example, most of my friends do not need to know how Riemman integrals are defined, but they do need to understand how to calculate derivatives, integral etc. If in any case your friend does not need to understand such proofs, then probably Rudin is not the best choice at the moment.
A second thing is that the writing style of Rudin quite peculiar (although not unique) to mathematics. It largely comprises of definition, lemma, theorems, proofs and examples, while lacking sometimes, for example, motivations and intuitions. One consequence of this style is that the writing is rather dense, and certain concepts only start to manifest their importance long after they are introduced. Another consequence is that proofs are rarely given intuitions before they are presented. All and all, this means the readers cannot simply read the books line by line as with many other textbooks, but have to seek attentively the intuition behind certain definitions and propositions. This also requires the readers to look back, and try to organize the material in their own way so that what is written on the book is no longer just disconnected facts. Unfortunately, doing so requires time and training and the process is rather personal. Luckily there are many resources to help, such as those pointed out by the answers before. I have also found that discussing with friends (with similar mathematical maturity) extremely useful.
Finally, there are sometimes certain writing styles that are particularly easier for certain people to understand while not others. Personally, I have found this much easier read to me, despite that fact that it is in certain sense more abstract. Again, this choice is rather personal so it would be worthwhile for her to first select the books that she is most comfortable with.
Hope these help