Mmmmm I know in the us army when the soldiers apply for us citizenship they don't not count the time spent a broad on mission for them to cumulate time residing on usa soil, mission ouside are considered as same as being inside the state. So I'm not sure if this is applicable to ur friend case. Coz if he served his country nepal under UN mission,he would be liabale under the nepalese law if he comited any crimes and its equal of spending time in his own country coz he's has the flag on his army uniform...
So for me he will only need a nepalees Police clearance.
That's just my opinion.