Its been said: VirtualDub is what she needs
What you gotta do is load the video, select Direct Stream Copy for it or use compression if you wanna re-compress it. Then for Audio, select Wave File and load the wave file with the voices on it. Use a program like Cooledit if you wanna mix the voices with music for instance, or any other background sound for that matter. Make sure the wave file syncs with the video.
Good luck
