This is the time of year when all the shore power cords seem to start melting. A combination of high current
use (heaters) and damp.
I was on the boat, exactly this time last year, when the cord melted down - fused solid with the receptacle.
What happens is that the connections get damp, corrode a little, the high resistance creates heat, and from then on it's a viscious cycle where the heat causes faster corrosion and so on.
At this stage you need to replace the termination on the shore power cord or the whole thing, and probably the receptacle on the boat.
Around here most people reckon on a 3-5 year life for shore power cords.
Secondly, your friends 50A cord has a ground fault.
Thirdly, your friend is using too much current
if he made the breaker trip with a 30A cord.