Could anyone recommend something that she could read that would give her an overview of what is going on in the Liturgy and perhaps a layman's explanation of some of the major symbolism involved? (e.g. the Great Entrance also representing Christ's entrance into Jerusalem, etc.)
Hate to be blunt, but this particular "symbolism" regarding the Great Entrance you mention, as revered as it is amongst many lay Orthodox (and unfortunately among many others who IMHO should really be looking into things more deeply) is simply wrong
, despite it being a pious opinion handed down by Orthodox Christians for hundreds or even thousands of years!
The allegorical interpretation of liturgy was somewhat unwittingly set in moition by the great St. Maximos the Confessor. I am not saying that people like him and St. Germanus and St. Nicholas Cabasilas do not have some useful things to say in this regard, but this allegorical approach is not espoused in the vision of liturgy held by the early Church and can sometimes in fact be contrary to it. At best, the allegorical approach does not get at the heart of what the liturgy is all about. What are, then, the essentials? I believe that what one has to remember is that during the litugy, the Church ascends to the Kingdom of God. It is only there that fallen material from this world can truly become the body and blood of our Lord Jesus Christ. In a very real way, the Church "becomes what She is" during the liturgy, and her sanctified members have to go forth from it and bring the Kingdom which they have received to others. "We have seen the true light! We have received the heavenly Spirit!" is sung in the slav version at the end of the liturgy (not sure if the current Greek usage has this, but anyway.) Only in the Kingdom of God can we receive the Spirit. The Kingdom has been inaugurated by the risen and ascended Lord, it is here, now, in our midst, and yet paradoxically it is still yet to come. The fallen world is still all around us, and having "received the heavenly Spirit" we have to go forth and proclaim the Good News of the the Kingdom to others!
So there is no "secret code" or some kind of "symbolism" that you have to uncover. (I put this word in quotations because its true, ancient meaning has been lost, and is only now being rediscovered.) The liturgy speaks for itself. The best thing to do is to go and pay attention, and try to view it all in this eschatological light I have referred to. This is what really explains what is going on. For example, at the so-called little entrance, when the priest blesses the entrance and intones "blessed is the entrance of your saints, always now and ever and unto ages of ages" what is he saying? On the one hand, it simply relates to a time when the entire congregation was entering the main body of the church from the narthex. But in a very real way it refers to the ascent to the Kingdom as well. Which part of the liturgy is most important, you may ask? All of it is, because it is all part of this ascent to the age of the Spirit.
If you or your friend still want to better understand the meaning behind what liturgy is all about, I would recommend reading For the Life of the World
and other works by Fr. Alexander Schmemann. IMHO, those who criticise his approach simply don't understand what he is talking about, or are taking him to task for a couple of things he said at times that were ill-advised. He has had a tremendous influence across confessional boundaries regarding what liturgy is all about, despite the fact that many simply do not "get" what he is trying to say. For a view of the historical development of liturgy, you could read things like Robert Taft's The Byzantine Rite: A Short History
and Hugh Wybrew's book on the Orthodox liturgy. I don't think this is what your friend should look at right now, but you might benefit, I don't know. Remember, however, that Taft and Wybrew offer valuable insight but are not Orthodox themselves, though Taft is Eastern Catholic.