  Most massive galaxies are thought to contain a supermassive black hole in
their centre surrounded by a tenuous gas environment, leading to no significant
emission. In these quiescent galaxies, tidal disruption events represent a
powerful detection method for the central black hole. Following the disruption,
the stellar debris evolves into an elongated gas stream, which partly falls
back towards the disruption site and accretes onto the black hole producing a
luminous flare. Using an analytical treatment, we investigate the interaction
between the debris stream and the gas environment of quiescent galaxies.
Although we find dynamical effects to be negligible, we demonstrate that
Kelvin-Helmholtz instability can lead to the dissolution of the stream into the
ambient medium before it reaches the black hole, likely dimming the associated
flare. This result is robust against the presence of a typical stellar magnetic
field and fast cooling within the stream. Furthermore, we find this effect to
be enhanced for disruptions involving more massive black holes and/or giant
stars. Consequently, although disruptions of evolved stars have been proposed
as a useful probe of black holes with masses $\gtrsim 10^8 \, M_{\odot}$, we
argue that the associated flares are likely less luminous than expected.
