Willy,
So your friend thinks it is burning oil because it sat for three months? Hmm, maybe, but I would be looking for a coolant leak. Either a blown head gasket or cracked head. Oil burns with a distinct blue smoke, water or coolant is white. I believe brake fluid burns white as well (being glycol based). Make sure you eliminate that possibility first by disconnecting and plugging the brake booster vacuum hose.
You don't HAVE to remove the engine, although it does make it tidier to work on. You could just remove the manifolds and head with the engine left in the car, and if the cylinders are OK, replace the head gasket. But you MUST have the head checked (dye penetrant or mag fluxed) for cracks. Opel heads are somewhat notorious for cracking at the exhaust valve seat if they are overheated even a little. I currently have TWO cracked heads, and they are generally not repairable. But used heads are fairly readily available.
If you do want to remove the engine, the Factory Service Manual says to drop it out the bottom, complete with the transmission. And so do I. A few on the list have removed the tranny and bell housing first, and usually the head, and lifted the block out the top. But it isn't very hard to drop it. Just raise the car 24 inches off the ground with a set of jack stands on 4x4 wood cribbing, measured at the front jack point, and 20 inches at the rear jack point. I had to replace a bunch of seals and gaskets on Katheryn McCoy's GT last winter, and I had the engine out and back in less time than it took to replace the seals.
A bit of a warning. If you do remove the engine, it seems like a "good idea" to do other things while you are there. And it is. But the list can grow quickly, to such things as a new clutch disk, a new rear engine seal, drilled and heli-coiled exhaust manifold threads, a new front engine seal, and more. Your friend might want more than $40 if you end up doing a fairly major repair.
Good luck!