The F1-visa that ur friend has is just to enter here. Once u r here, ur status is maintained by college status (I-20). If ur friend has changed to F2 status, then his/her F1 is automatically cancelled. It is not that how ur fren know F1 status is cancelled, it is autotamtically done one u r in another status. So the expiry date of F1 visa printed on passport is no longer valid.

Ur fren now can remain in US in status till the primary F1 holder (under whom ur fren has changed to F2 status) is in status. During this time, ur fren can apply to college, get I-20, and change status back to F1. But if ur fren goes to nepal, then he/she needs to apply to college, get I-20, appear for F1 visa interview and if got it, then can come.
Even to come on current F2 stattus, ur fren must appear for interview to get F2 visa as F1 visa on passport is no longer valid.

So the point is if ur fren goes to nepal, interview is mandatory no matter on which visa F1 or F2 he will be returning to.

So its better to join college over here, change to F1 status and then only go nepal and come.