Is swearing by other than Allah considered disbelief, and is obeying someone who swore by other than Allah concerning a specific matter also considered disbelief?

If your friend makes you swear by other than Allah to do or not do something, then you must forbid her from this evil and clarify that swearing by other than Allah is forbidden in Islam and is a lesser form of disbelief (kufr dūna kufr). She must atone for that by saying "Lā ilāha illallāh" (There is no god but Allah). You do not sin by obeying her in what she requests if it is permissible, nor is that considered disbelief. However, you do sin by neglecting to forbid her from the evil if you fall short in that.