Would a random passenger be allowed to fix a problem with a commercial aircraft? I just read an article about a man (who is alledgedly an airplane mechanic) who was a passenger on an airplane that had some mechanical issues. Because there were no 'local' mechanics around, he decided to take matters into his own hands and fix the problem himself.

Here's the article I read (it's in Dutch though)

Here's one in English

This all seems nice but in this day of age where airplane security and regulation is so tight, I wonder if this is possible at all. It also seems to me that by the airplane company is taking a huge risk in letting this "random" person fix this problem. The article obviously oversimplifies a few things, but regardless of that, here's my question:

Is it possible for a random passenger, who also happens to be an airplane mechanic (working for airline X) to fix an issue on the airplane he happens to be on, given that there are no "local" mechanics around (however unlikely that might be)?
I happen to have personal knowledge of this particular case. I was in that airplane, too, seated two rows away from him. I'm also a former pilot and was naturally very interested in what happened, so I asked him some questions during the flight back.

He had worked as service mechanic for this company before, servicing their planes in Iceland. Their technical staff and the pilot of this plane already knew him, which made things easier.   

Tools and parts were available at the airport in Alicante, but no qualified technician certified for this type of aircraft. The technical staff of the airline contacted his boss in Iceland by phone, who agreed to sub-contract him to the airline for the duration of the repair and pay him overtime for the work.
They faxed a copy of his certification documents to Stockholm to cover the EASA regulations that require a certified professional doing the repair.