As long as he has a valid re-entry visa in his passport then the travels should be fine. If he needs a new reentry visa from the embassy in Kathmandu that might be riskier. Do make sure your friend gets a new signature on his I-20 though. (The signatures don't last as long on OPT as they do while one is a student). During my past three re-entries into the US on OPT I only needed to show my passport (with valid visa) and I-20, endorsed for OPT. One is required to have the employment authorization card and a letter from the employer along, but they generally don't ask for it.