Three questions:
a) When the engine is dead-cold, and you release the coolant cap, do you get a *POP*? b) When you are done with a moderately long drive (that is, the engine reached full operating temperature for at least 10 minutes), put the car in the garage and allow to get cold as above (overnight). When you remove the oil-fill cap, is there any grey/white/pale-brown foamy sludge on the cap... about the consistency of warm vaseline? c) What does your temperature gauge do? Does it cycle between hot and cold rapidly?
If yes to the first, you have either a blown head-gasket or a cracked head. Either way, some oil and coolant may be being blown out your exhaust.
If yes to the second, you are getting coolant into your oil via some mechanism which then reacts with the oil to form the sludge. This last test must be done only after the engine has been run at full operating temperature for sufficient time to clear all the natural condensate from cold starting out of the oil. So do not be overly alarmed if you see such sludge after a series of short trips with the engine cooling between each one. "False Positives" are common.
Lastly, it could be that your thermostat is malfunctioning. If your engine is running cold, fuel consumption will increase greatly. If the stat is short-cycling, then excess pressure will build in the engine loop, blow-off coolant and so forth. The engine will overheat a bit and use excess oil, then cool suddenly when the stat does open. All of it simply *not good*.
If "NO" to all of the above, then fill to nominal levels and keep a real eye on it. Betcha your friend drove at very high speed at something of a load. Oil consumption is logarithmic... the first bit takes a long time to use, the second bit much less, and so forth as less and less oil is in the sump to handle heat and wear.
But in a complete vacuum, from a distance, not seeing the car and given its approximate age and condition... I lean to the T-Stat.
Peter Wieck Wyncote, PA