  Context. The extremely low density of several long-period exoplanets in
mature systems is still unexplained -- with HIP 41378 f being archetypical of
this category. It has been proposed that such planets could actually have
normal densities but be surrounded by a ring observed approximately face on,
mimicking the transit depth of a puffy planet. This would imply that the
equator of the planet is nearly perpendicular to its orbit plane, which is at
odds with the formation process of gas giants. Yet, in the context of the Solar
System planets, it has been shown that after gigayears of evolution, the tidal
migration of a moon can naturally lead to a very tilted planet with a ring.
  Aims. As exomoons are expected to be ubiquitous around giant exoplanets, this
mechanism may be responsible for the anomalous radii of some observed
exoplanets. In preparation for the future discoveries of the PLATO mission, we
present a simple method for checking the plausibility of this mechanism for a
given exoplanet.
  Methods. Analytical formulas give the probability density function of the
relevant precession harmonics of the planet. For each harmonic, simple criteria
set the moon mass and other properties required for the mechanism to operate.
  Results. We applied this methodology to HIP 41378 f, and we show that in
order to reproduce the observed configuration, a hypothetical former moon
should have had a moon-to-planet mass ratio of a few times 1e-4 (i.e. roughly
the mass of our Moon) and have migrated over a distance of a few planet's radii
on a gigayear timescale. These orders of magnitude match the properties of
moons expected to exist around gaseous exoplanets.
  Conclusions. We conclude that the migration of a former moon is a viable
formation pathway for the proposed ring and tilt of HIP 41378 f. This example
strengthens the ring hypothesis and motivates its application to other targets.
