I think the rule of thumb (for any ship purchase/upgrade) should be that if you can do it better (whatever it is you want to do) in your current ship then don't upgrade (or do downgrade if you are finding out the hard way).
For example if you want to make money doing combat in res zones and you do it better in a Vulture then do that until you can equip your next ship better (or not at all). I know players making far more in a Vulture than I did in my Python.
Same goes for trading, although providing you can carry more cargo and you aren't making less cr/hour then it probably matters less.
I appreciated the Python more once I had a decent psu, fsd and shields in combat but it made far more cr/hour trading than my Clipper - Which then allowed me to upgrade it for combat more quickly.
I think a good way is to mix it up. If he likes a spot of trading, the python is perfect. And he can continue fighting in the vulture until he has enough to upgrade the python.