Lubricating the cable will make it worse! Modern bowden cables have a plastic/nylon liner which will swell if oiled, making it tighter.
You could try fitting a new cable, they're about £25 from eurocarparts.com or gsfcarparts.com, probably about the same from local motor factors, but I dont think that'll solve the problem. A worn cable is usually hard to push down and jerky to release. If your wife is finding it hard to 'hold' the clutch, that's just the way peugeot clutches are.
Might be easier to get a Stairmaster and build up her leg muscles!
trent, August 2009