Can outer tyre (casing) punctures/cuts be fixed? My flatmate got his tyre stabbed. With a knife, it seems. (The bike was tied to a staircase railing in the building and was obviously in some maniac's way. Though there was plenty of room around. And another bike next to it.)

Is there a way to fix the cut (it's quite big, some 2 centimetres)?

EDIT:

Here are some photos: https://www.dropbox.com/sh/oulsuugvjaihta2/a3tiZm2BEp
You can "boot" the tire.  This involves placing a piece of relatively stiff material inside the tire to cover the hole.  For smaller holes a piece of US currency (which is printed on quite durable paper) is often folded several times and inserted, but 1cm is about the limit for that.

You can buy a commercial "boot" that is a piece of plastic or stiff rubber designed for this duty, but they're a bit expensive and really only intended for cyclists to carry for emergency repairs on the road.

If you can get your hands on a scrap bicycle tire you can cut a boot from that.  Generally you'd cut the piece from the sidewall, not the tread area (because the sidewall is thinner), and for a 2cm cut in a tire you'd make the boot at least 6x6 cm, and ideally as large as you can manage.

You insert the boot in the tire, maybe with a spot of tire patch glue to keep it from shifting (but make sure the glue doesn't ooze into contact with the tube and glue it in place), insert the tube, mount, and inflate.  Partially inflate first and check that the boot appears to be centered over the cut and not bulging, and that the tire is properly seated on the rim, then, when you're confident, fully inflate.

When you use a boot like this there will always be a "lump" in the tire.  And the repair is nowhere near as reliable as a "sound" tire.  But, properly done, this can be quite serviceable.