Is it true that someone with a lumbar hernia can lift max 30% of his weight? if not then what is a safe amount of kg to work with at the gym? thanks!

No, not true at all: Lumbar disc herniations are actually very common and can be found in people with no back problems at all. So simply having a hernia does not necessitate any lifting restriction; the restriction depends very much on symptoms and varies widely from person to person. Generally speaking, avoid activities that cause pain and slowly, cautiously increase exercise capacity within the limits of comfort.