Welding steerer tubes
It appears that it is possible to extend a steel steerer. In this thread on Velocipede Salon, one person posted a video from a framebuilder, and one other person I assume is a framebuilder also responded that it is possible to weld an extension on to the tube. It may also be possible to replace the steerer entirely.

You should consider if simply buying a new fork would be more economical. Alternatively, depending on how far you cut it, it might be possible to achieve a similar riding position with a riser stem.

Your post mentioned two people with "dh" rides, which I assume means downhill mountain bikes. I am not familiar with welding, but I'd expect that welding your steerer might reduce the steerer's strength. Also, I'd expect that an improper weld would have a stress riser that could eventually break. You would want to confirm if a repair is wise with your friend. Trusting the repair to a framebuilder might be a safer option. And I'd also expect that downhill mountain biking will stress all parts a lot more than road cycling will, so it may be better to just get a new fork after all.