Tanis said:
Just to be a bit of a nagger...
-Let the Right One In-
It's not stated, at least in the film, if Eli is actually a female.
Yes, Eli is played by a female actress.
But, then again, so was Ash in Pokemon.
Also the vampire isn't necessarily doing what it is doing to be the boy's friend. If you think about it the vampire cleverly manipulates the boy into becoming his new ward by getting him to fall in love.
There is one short scene (the one where the boy looks through the cracked door when Eli is changing) that eludes to this particular thing, but the book this movie is based on (same name by John Ajvide Lindqvuist) goes into greater detail:
!!!!!!!!!!!!!!!WARNING !!!!!!!!!!!! SPOILERS FOLLOW !!!!!!!!!!!!!!!! WARNING !!!!!!!!!!!!!!!!!!!!!
Eli is in fact a male whose genitals were mutilated by the sadistic vampire who turned him. It isn't shown in the movie and the American remake made Eli into a girl which was an absolute fail in my opinion. It also further details the relationship the vampire and the pedophile have.