I would think much depends on their reason as to why they choose to not talk much.
Reserved - because they are unsure how their comments may be taken, or are self conscious.
Shy - because they are uncomfortable.
Introverted - because they don't like being noticed.
Bored - because they don't like the conversation subject.
A second language - because the conversation is unnatural and are not confident of their speaking or listening ability. (I apologize if this is incorrect or offensive.)
Sometimes people like to listen and are observers.
These are my thoughts and perhaps the definitions may be interpreted a different way.
I agree with "reserved", "shy", or "introverted". You could even say "She is a bit of an introvert!" and people would know exactly what you mean. I even think that saying "tight-lipped" would be a bit odd. To me, that kind of sounds like your friend is hiding a secret or holding back from saying something that is on her mind :) But, in essence, that would mean that a person is quiet.
"Reticent", "laconic" and "taciturn" all sound like you are reading out of a piece of 18th- or 19th century literature. Surely you could use these in an academic or literary paper, but saying this in casual conversation would seem a bit out of place. I think the last time that I actually remember having to keep "laconic" and "taciturn" in the forefront of my lexicon was when I was studying for the SAT and LSAT ;)
I can offer you a beautiful phrase that the English use about quiet people:
"still waters run deep"
It is a compliment that means people who are calm and quiet are usually very intelligent and deep in their way of thinking.