That's a bit like saying "sprained ankle, going to amputate left testicle.' Most likely blown head gasket as others have mentioned. Unless you drove it like that for a long time and there's evidence of other internal damage, a rebuild is a stretch.
The coolant breaks down the oil and the liquid becomes thinner than oil. In turn if driven like this for a while without proper lubrication in the block, it will cause damage. This "fresh 308" could be in trouble.
I'd suspect whoever did this engine didn't properly torque the heads down in order, in stages. That's just a guess seeing this car doesn't have forced induction.
That sucks man. Drain the oil well, like let it drain for a day. I hope the block isn't hurt.