99 Corolla engine block oil leak

As far as I know, the engine block is one piece. Since there is no silicone between the engine block and the head, I am assuming that you are talking about the seal between the block and the oil pan gasket. Hopefully, your friend checked the bolts on the oil pan to make sure they are tight. Sometimes, a little tightening will stop the leak.

The oil pan has what is known as a formed-in-place gasket (FIPG), using a special caulk that does not affect O2 sensors. Regular silicone caulk will damage O2 sensors. If tightening the bolts doesn't stop the leak, then the oil pan has to be removed, the old FIPG scraped off of the block and the pan, and then new FIPG applied and the pan tightened evenly. Fortunately, this is not a huge job, it is more labor-intensive than parts intensive.

As far as I know, there is no seal or gasket between the block and the main bearing caps. The crankshaft and the caps that hold the crankshaft ("main") bearings are mated to the bottom of the engine and are enclosed by the oil pan. The engine has a front and a rear crankshaft seal that could potentially leak, those are the only seals near the main bearings that could leak other than the oil pan seal.

To be honest, I am not that familiar with the Corolla engine, so I suppose that it is possible that the bottom end of the engine is cast as a separate piece, but it seems that it would add unnecessary complexity and cost to the engine assembly process.

The only other potential sources of oil leaks that I can think of are the oil pressure sender, head gasket, valve cover gaskets, camshaft end plugs, distributor, oil filter mount, and dipstick tube gasket.

Good luck with the repair!