Is it considered a grave sin to undergo an abortion, and is helping a friend abort her fetus, which is not more than five months old, considered participation in the sin? If so, what is the way to expiate this grave sin?

You made a clear mistake by helping your friend abort the fetus. Your duty was to advise her to repent from the indecency and not to abort the fetus. You must perform sincere repentance and never return to such an act. It is impermissible to abort a fetus at any stage except out of necessity, especially after the soul has been breathed into it. If you participated in the abortion, then you must share in paying its diyah (blood money), and kaffarah (expiation) is incumbent upon you.