She is being friendly中being是表示什么意思?is不是be的一种形式吗?为何在这里又用is,又用

First let's compare these two sentences:
1) She is smart,hardworking and friendly.
In this case,"She is friendly" means that she is a friendly person (she is always friendly to other people.Her friendliness is her nature).
2) She is a very aggressive person.She does not treat her coworkers very nicely.But today,she is being friendly to me.
In this case,"She is being friendly" means that she is friendly to me at this moment.But this does not mean she is always friendly to me.She may not be this nice tomorrow.
In short,"She is friendly" = "She is a friendly person." "She is being friendly" = "At this moment,she appears friendly to someone."