Searching the threads seems to confirm that both are correct. It's all covered in the Ref.Guide.
Seems acceptably thematic to ask to your ally to join a co-investigator.
A little less thematic to find the ally "in a suitcase" of a defeated investigator at the hospital or asylum. Though I suppose maybe you would find their contact info and they are willing to join you.
Can someone confirm if both are correct? In the past, we played they were tradeable but left when their owner investigator was defeated.
• An investigator’s possessions include his Clue tokens, travel ticket tokens, Asset cards, Artifact cards, and Spell cards.
• As an action, an investigator can trade any number of possessions with another investigator on his space. The two investigators may freely give any number of possessions to the other, as long as both investigators agree to this trade.
• When an investigator has lost all Health or Sanity, he is immediately defeated and resolves the following steps:
3. Collect Possessions: The investigator discards all Condition cards, Health, Sanity, and Improvement tokens and places his possessions on his Investigator sheet. Keep the sheet faceup and place it in a common area, out of the way.