I don't know what year your friend had, but from '94 on the car went through a major redesign and most of the reliability issues went away.
My Bro-in-Law has an '88 XJS C (V12 2-seat convertible), and I agree, he's had issues with it. He lives 800 miles from me, so I can't help him with it, he takes it to a Jag independent, not at all mechanically inclined.
Btw, it's a misconception - British cars do not leak... they just 'mark' their territory
This one though is totally dry, except for some seepage at the waterpump gasket and the leaky shaft seal on the diff - we're waiting for the parts to repair them.
My buddy really loves this car, says he's wanted one since he first saw it 15 yrs. ago while in school. I hope it's good to him, but in any event, he's got the money to deal with it (he owns a software co. and his wife is a physician) so it's all good.