We use comparative adjectives between two things. Why in this sentence we use "other than the others? Isn't that tree bigger than the others? Do we say "my brother is shorter than me or I"? She is taller than her friend (is) can "is" be used here or not?
In the first, use other if there is one other tree; use others if there are two or more other trees. Even though comparatives are between two things, this construction can be used for several things because you are comparing your tree to each of the other trees individually. If you asked about the group, you would say "Which is the biggest tree?"
Technically, because "than" is a conjunction, it should be followed by a clause. So, it is preferred by many to use "I" in "my brother is shorter than I". It is an ellipsis of "my brother is taller than I am". In informal English, many people use "me". That turns "than" into a preposition, which is considered a usage problem by careful speakers and writers.
Yes, you can use "is" after "her friend".