It's probably trifoliate orange/hardy orange (Poncirus trifoliata, or, depending on which plant taxonomist you talk to, Citrus trifoliata), although the fruit looks rather weird, so perhaps it's a hybrid with another Citrus species. Have your friend cut open a fruit -- it should look like what you'd expect a citrus fruit to look like inside)
Trifoliate orange can be pruned into a somewhat attractive specimen plant:
but it's likely to be extremely nasty work, as you can see from your photo, and typically it just grows into a great blob of horrific thorns:
Trifoliate orange is native to eastern China and Korea, and, at least in the southeastern United States, it spreads relatively well by seed and can sprout into dense thickets, and it is becoming somewhat of a pest (and by dense thickets, I mean "can replace fences around highly secure areas such as airports and nuclear facilities" thickets). I'm not sure how it grows in a hot, dry Mediterranean climate, so maybe it is better behaved there. Still, if it was on my property, I'd collect a bunch of the fruit and try to make marmalade, maybe keep a sprout or two to experiments with as a potted tree, then dig up the rest with extreme prejudice.