Usually the webbing at the chunk is a good identifier but since your buddy is not familiar than look for the casting marks in the lower right web corner when you are facing the diff cover. MOST of the time it will tell you what model it is but not always.
Just some guide lines:
"B" series D70's usually have a large B casted on top of the pinion channel of the chunk.
HD D70's are just stout as can be and look like they are on steriods - even id you are not familiar with axles a HD 70 will look beefier than others.
"U" series D70's are very smooth on the bottom and almost round in shape on the bottom side.
If still in question post up a pic for your buddy and we'll do what we can to help .
HTH . Tom