When a meditator achieves e.g. the 5th jhana of infinite space, is he
or she still in this world?
Yes, he doesn't disappear. When you sleep, you are not aware of what's happening around you. But that doesn't mean the world is gone.
Obviously the body is, but what about the mind?
Yes, the mind is still associated with the body. Just that it's focusing on the space element.
can sentient beings perceive the world of form as something formless?
Atoms are 99.9999999999999 percent empty space. They say If you remove all the space in the atoms, the entire human race could fit in the volume of a sugar cube. That gives you an idea of how much our senses fool us. It's mostly empty space.