I said I'd sort out disc brakes on a friends bike,but they are hydraulic and I've pretty much no experience.
Anyway,the problem is that when braking hard,the front one screeches really loud. Any ideas what could be the cause? Could a leak in the hose cause it by letting fluid on the disc/pads?
Any suggested solutions would be appreciated.
Thanks.

If you take out the pads and they seem stained with something and if you can see traces of it on a piece of toilet tissue pressed against the pads for about 5 minutes then yes. Otherwise it could be something that splashed up onto the discs and contaminated the pads.

Start with cleaning the pads with a paper towel soaked with a few shots of brake cleaner spray solvent from an autoparts store. Clean and repeat until the paper towel doesn't come away with any blackness on it. Spray down the pads as well just because you've got them out where you can see them. Let dry in the sun until you can't smell any of the cleaner anymore. It won't hurt to use some of the brake cleaner to hose away some of the grunge in the caliper around the pistons. Use a bowl to catch the solvent for neatness. Again let dry before replacing the pads.

Sand the faces of the pads with something around 240 to 320 to remove the shiney surface. Don't go crazy, just make the faces look dull. Put it all back together and go for a test ride.

If they still squeal then sand the rotors using some 320 wetordry sandpaper (the black stuff) using water and dish liquid to scuff them up a little. Use a backing block of wood or metal or whatever so you're sanding them flatter rather than following any dips and curves. Wash well and do the brake cleaner and paper towel thing again.

If that doesn't work sell it and pass him off the money less a commision fee.....