My party encountered a Mindwitness that was abandoned by its Mind Flayer masters. Now that its abandoned Volo's Guide will seek out another creature with immense psionic power, before it finds one is it reasonable to say the party can convince it to work with them for a while? What might a Mindwitness desire in exchange for its help?

Mindwitnesses are imprinted with a need to serve and obey commands, so in the absence of it's original community, it would definitely try to join a group that it could aid. However, it seems to primarily recognize telepathic creatures. If the group had a telepathic character, then convincing it to join the group ought to be fairly easy.

Since service is a biological imperative, it wouldn't desire anything in exchange for its help. It is compelled to serve.