Zuki sidekick with 16v. Mech. opinions needed!

Lady bought with 30k miles. Now has 106k. Was driving last week and it overheated so she brought it to shop. Radiator was empty and the thing still ran perfectly. They replaced upper and lower hoses, thermostat, flushed and re-filled coolant and then told her she may have to replace headgasket as they saw oil and coolant mixing. (what kills me is that they hosed her and charged her $375 for this work that was obviously not needed if the other problem existed!) So I bought the vehicle. Upon inspection I noticed that rad was empty and she was surprised to hear that. Vehicle started up and ran perfectly and did not stumble at all. I filled up with coolant and made sure there weren't any air bubbles in system. Drove home and ran like a top. Checked coolant and it was empty again! Was not burning a single drop nor was it leaking anywhere even when the engine was under load. I filled a little more when I discovered that crankcase was filling up with coolant!!!! Now my tech question is this....Problem was before possible headgasket failure as coolant was already disappearing into crankcase which caused initial overheating. Also...there isn't any pressure whatsoever in coolant system. knowing those two things....how do you think the coolant is getting into the crankcase? Any and all suggestions are appreciated! I really want to fix this thing due to the fact that it is actually in cherry condition!!