{coi} is a cmavo of COI group and allows a name or a selbri after it. If it is a selbri, like in this case, then it is treated like {lo} plus that selbri. In our case, greeted is {lo lojbo tadni}, i. e. a Lojbanic student. While this could also mean that the student just applies some Lojbanic methods for studying, from the context it is clear that the person studies Lojban. More correct would have been {tadni be la lojban.}, but everyday language prefers shorter ways.
A: How did you know I am a Lojban student?
B: You greeted me in Lojban ;)
do rinsa mi bau la lojban.
A: Yeah.
go'i
B: What country are you from?
ma gugde do
A: I am from Canada.
la kanadas.
B: I see. I am from Germany.
je'e .i mi se gugde lo dotco
A: I like German beer.
mi nelci lo dotco birje
B: I don't drink beer.
mi na pinxe lo birje
A: You don't drink alcohol beverage at all?
B: No. Only beer, I don't like its taste.
A: So, you like wine?
B: Very much.
mutce go'i
A: My wife likes wine, too.
si'a lo speni be mi cu go'i
B: Glad to hear it.
A: I miss my wife greatly.
B: What's happened?
A: I am on a mission in other city, and will see her only in two weeks.
B: Oh.
A: My wife is very beautiful.
lo speni be mi cu mutce melbi
B: Glad to hear it.
A: She has long hair.
lo clani cu kerfa sy.
B: I prefer women with short hair.
A: I find nothing attractive in women with short hair.