Context will help on some of these. If we are waiting for a particular person to show up - Mary's friend, for example, but you have not met Mary's friend before -- and you say, pointing to the person walking through the door, "Is that her friend?" I'd say "Yes, that's her."
If we are looking at photographs of Mary's trip somewhere that she took with a friend, you point t
You should use "Yes, it is" in that context. The word "it" refers to "this (person)".
In addition to what Barb has written, there is no indication in your question ("Is this her friend?") whether the friend is female or male. It is entirely possible that the friend is male.
"This is" is not written as a contraction, but in everyday speech it will often s