A friend of mine who works at Guitar Center and knows worlds more than I do about guitars/amps recommended I buy an Egnater cabinet.

They're like $600 or so, and come loaded with Celestion V30s.

He also said they're made from marine birch, just like Orange cabinets. Is this true? If so, does this mean it will sound the same as an Orange cabinet (with a different head, of course)?

Sounds like a hell of a deal to me.

Agile AL-2000 > Maxon OD808 > Boss DD-20 > Strymon blueSky > Splawn Pro Stock > Emperor 4x12

the lives to come

Egnater has a long standing reputation for building quality stuff, and I have no doubt that it's a quality product.

It will not sound the same as an Orange cab. Simply put, the specifics of the size and shape affect the tone of the cab, not just the construction materials. The Avatar Contemporary cabs are almost spec for spec replicas of the Orange cabs, so if you're looking for that specific tone it may be worth looking into.
ESP LTD EC-256 and a Fender Deluxe VM