What is the make, model, year, and engine size of the vehicle?
Ther are only a few possible things that can allow engine coolant to get into the oil:
(1) leaking head gasket or intake manifold gasket
(2) a crack in a cylinder head casting
(3) an internal crack in the engine block.
Of these things, a head gasket leak is the most common cause, but cracked cylinder heads are not unheard of on Ford 6 cylinder engines, particularly if it has ever overheated.
Unfortunately, it can be very difficult to positively identify the exact place that the problem is occurring since teh leak is on the inside of the engine, and all of these causes have teh same symptoms. To determine exactly where the problem is occurring, your engine will need to be disassembled for a visual inspection.
Engine disassembly and replacement of the faulty component is the only reliable way to repair this type of problem. You do not want to operate the engine with an internal coolant leak, as the coolant leaking into the oil will result in damage to the crankshaft, engine bearings, cylinder walls, and other internal engine components.
Ther are various stop leak products on the market, but these are not a repair and at best only temporarily slow down the leakage while causing other problems such as heater core and radiator blockages, so use of stop leak products is not a recommended fix.
I would suggest having your mechanic disassemble and rtepair the engine before driving the vehicle any farther. Yes, this is a rather expensive repair (cost varies widely depending on what exactly needs to be repaired or replaced inside), but it is far cheaper than having to replace the entire engine becase it was driven a little too long with the leak.