My opponent (we're both beginner players) casts a Renegade Freighter,
which is an Artifact — Vehicle. Can he attack me with it on his next
turn?
Yes, if it's a creature. A creature can attack when it has been in play, under the same players control since the beginning of the turn (or if it has haste), even if it wasn't a creature previously.
Also, it says "Crew 2", which, when enabled, turns it into an artifact
creature. What is an artifact creature?
Crew X is an ability, when activated adds creature to its types.
So the card by default reads:
Card Name:
Renegade Freighter
Types:
Artifact — Vehicle
Card Text:
Whenever Renegade Freighter attacks, it gets +1/+1 and gains trample until end of turn.
Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)
P/T:
4 / 3
After a player activates Crew 2 the card reads:
Card Name:
Renegade Freighter
Types:
Artifact — Vehicle Creature
Card Text:
Whenever Renegade Freighter attacks, it gets +1/+1 and gains trample until end of turn.
Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)
P/T:
4 / 3
This makes the card a creature, so if you have a card that says Kill Target Creature, you are now able to target Renegade Freighter.