my friends windscreen blurs when it rains and any wipers she uses does not sort it nor does cleaning the windscreen?

Hmmm, this is a strange one. For starters, stay away from rain-x and high end washer fluids for now. Yes, the tank may be contaminated with a wax type cleaner or additive.. Rinse it out thoroughly with something strong. Then flush with water. Take a razor blade to a small area of the windshield ans see if you find a film of some sort.

Toothpaste and vinegar will really clean the scree. if it's still blurred it must be the wipers aren't making sufficient contact. Try the flat wiper blades, they are floppier and conform to the windscreen better.

You clearly have a greasy windscreen. Wet the screen and with some neat detergent on a cloth thoroughly scrub the entire windscreen. Do the same for both wiper blades as they aren't worn. Then wash the soap off the glass and wipers. I usually have to do this two or three times a year. OH, if you use hand pressure washer at a garage to wash your car, DON'T use the hot wax setting on your glass. It's fine for the bodywork(although it was wasted on my muddy Land Rover). After using the wax setting on the glass I had the same effect as you describe and it took several attacks with cellulose thinners to get rid of it.

Once you have it cleaned, I do recommend applying Rain-X every two or three months. When it rains the water forms balls and just rolls off the glass and it does seem to help resist ice sticking to the glass during the winter too.

Try getting some RainX - thoroughly clean and degrease the windshield and then apply it. The water will literally sheen off.

Get some new wiper blades while you are at it.

Thoroughly clean and degrease the glass, and then do the same for the wiper blades. This latter is important, the blades often carry grease and crud they've wiped off the glass and simply re-spread it to create smears again. It's often overlooked. Try wiping the blade edges with paper, you'll be surprised at what comes off. I always keep a sheet of fresh paper towel in the car for that very purpose, it's necessary with our roads and weather!

It may be of course that the wipers are old and worn out, but try the cleaning first.

I used to have that problem, until I washed the INSIDE of my windshield.

Use rubbing alcohol and newspaper to clean the windshield AND the rubber on the blades.

It is oil spray from the oily road surface. After a few days of dry, when it rains there is this oily film mixed in with the first rainwater.

If you were in a town where it rains daily the film is not evident.

Use Windex and hand wipe the windshield those odd days and the scum is reduced.

Clean it with pure rubbing alcohol, it the problem still persists, replace the windshield.In an emergency, rub a raw potato interior on the glass.

Clean the glass really well using white vinegar then apply Rain-X per the instructions. Drain the wiper fluid bottle and clean it out really well then refill with fresh fluid. That should fix the trouble. If it doesn't it probably needs a new windshield