Ford Mondeo 2ltr diesel 51 reg - lights dimming - bj56

a friend of mine has just bought the above said car and has asked me about it he says that every so often and when driving at night his headlights will dim as well as the instrument panel and his stereo will cut out its the standard ford stereo fitted then within seconds it will all work again any ideas will be appreciated thanks

Ford Mondeo 2ltr diesel 51 reg - lights dimming - Peter.N.

Sounds like either a faulty alternator or a bad connection to same, could possibly be that the belt is slipping.