Hmmm, could be looking at a HG replacement then.
Is this the first time it's blown the hose? Fill it back up, make sure you bleed it properly and keep your eye on it. Make sure your mate doesn't rag it everywhere until he's certain the HG is ok.
First signs of overheating/loss of coolant/mixing oil and water you'll know.
When my dads' old 820 had HGF (rad leaked while parked up, overheated on the motorway) he drove it for a few weeks keeping it topped up. When the pressure started building up, you could hear the water in the expansion tank boiling.
One time I was in the car with him, he couldn't stop straight away so had to keep driving for about 100 yds before he could pull in and turn the engine off... it was so close to blowing a hole in something
edit... So it's a GTi instead of a GSi? What age is it? If it's not a convo, it'll be worth getting it fixed, or even do it yourself.
If not, I'll have it