I have a friend who has a variegated lemon he said he grew from seed many moons ago. It now has fruit on it but he is not sure when the lemon is ripe - from reading other blogs it sound like the fruit is ripe when the stripes lighten up? My best guess is that it is a Eureka Lemon, the fruit is pink in the center.

The pink Eureka will turn almost completely yellow when ripe. Yow will still see some faint stripes...

yes, but unripe it will not only look better, but also taste better ...