Sure, it may be able to do 260hp on stock internals, but for how long?
Being an N/A car, im guessing it would'nt have forged internals. So therefore, the engine is'nt going to be no where near as reliable compared to if you fitted forged internals, which are made to take the extra strain that a turbo brings. Its completely upto the owner of course. He could either turbo the tired, old engine and drive around or he could do it properly, without cutting any corners. It would take longer, and more money but in the end, he would end up with a nice, powerful, reliable engine.
Imo, tristan is right. If you want to turbo a stock N/A engine, then a rebuild would be best.