That depends, do you want it to be "Sci Fi" or "Magic-ish"
For sci fi, she could get some kind of machine that stabilizes her molecules and gives them coherence.
Magic, Some magical item that does the same thing without bothering to explain how.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
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)