Hm...Are you looking for an ability, an item or an alternative solution?
Item: Magic- a fetish, a charm, etc. (may require an incantation or spell to work)
Tech- a machine, an electronic device, a tool (perhaps allows only temporary corporeality before needing to be recharged, or can keep her tangible so long as she remains in contact with it, or requires a certain fuel, etc. to work)
Ability: Solidification- power to give coherence to intangible, incorporeal or gaseous matter (including ghosts/phantoms)
Alternative Solution: 1) Medium with the power to channel voice and actions of spirits/phantoms (use her to interact with the physical world); 2) Medium whose presence causes ALL nearby ghosts/phantoms to become visible and tangible, so they can interact with their environment 3) Curse- phantom state only lasts for a certain amount of time, or only initiates at a certain time of day, or happens randomly, or is prolonged but not permanent, or is brought on by a particular experience/stimulus (like smelling blood, or experiencing fear, etc.)
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)