They probably couldn't get it off either and didn't bother to change the filter. In fact, it might be stuck because it hasn't been off in a while.
You might think I'm kidding, but it happened to me. I bought a BMW with a perfect service history (done at an independent shop). Receipts for oil and filter changes every 3,000 miles. Well, the first time I changed the oil and filter (after a mighty struggle with the cap), I find the oil filter had not been changed in years. It was completely full of dirt, so much so that the pleats were not even visible, just a solid chunk of dirt.
Get that cap off, one way or another. A new cap isn't that expensive, so if you have to break it to get it off, do it. Don't break the housing though. If it won't budge, you could always take a hack saw to it (careful not to cut the housing). It is probably stuck where the plastic lip meets the aluminum housing.
Good luck!