1 ) How long he will get time to look for new job ( H1b transfer ) if he decides to leave the current job and switch to new job. My question is will he be statusless immediately he leaves the job ?
… There will be absolutely no time for him if he leaves the current job and then start looking for another job. While he is working with current employer, he can start looking for another job and if he gets it, his new employer should apply for H1b petition for him. Only after receiving the H1B application receipt form (not necessarily the approval) from USCIS, he can start working for new employer.

2.) If he wants to change status back to F1 what is the procedure ? I mean do he need to do something from USCIS or go back to nepal for F1 stamping.
…..To go back to F1, again need to apply for university and get I-20. Once he receives I-20, then he can apply to USCIS for change of status from h1b to f1. Remember that he should be working with the current employer during the process.
F1 stamping or h1 stamping is needed only when you go out of US and need to come back. As long as u r in US, no need to think of stamping.