If the oil is too thin it's more likely to make it past the piston rings and be burnt (causing the blue smoke), especially at 120k miles where they will be a bit worn.

If you hadn't noticed blue smoke before then it's possible they did just use the wrong oil, or they've used the correct oil (5W30), and it's now too thin for the engine. Switching to a slightly thicker oil might help a bit, I had the same issue with a Mondeo and going to 10/40 improved it a fair bit, but if it keeps up long term the only final solution is really to rebuild the worn engine.

Don't despair yet though, it sounds like it could just be the wrong oil and you've nothing to loose by checking it out.