  The contribution of active galactic nuclei to the magnetisation of the
Universe can be constrained by knowing their duty cycles, jet and magnetic
field morphologies, and the physical processes dominating their interaction
with the surrounding environment. The magnetic field morphology and strength of
radio lobes of AGN has an influence on the mechanisms for the propagation of
cosmic rays into intergalactic space. Using the source B2 0258+35 we want to
investigate the interaction of its radio lobes with the surrounding environment
and examine the underlying physical effects. Published HI and radio continuum
data at 21cm were combined with newly reduced archival WSRT polarisation data
at the same wavelength to investigate the polarised emission in the radio lobes
of B2 0258+35 where we detected a unique S-shaped polarised structure. We
calculated the pressure to $p=1.95 \pm 0.4 \cdot 10^{-14}$ dyn cm$^{-2}$ using
an energy equipartition approach and investigate the physical processes leading
to the detected emission. We observe depolarisation in the northern lobe, which
might originate from the HI-disc in the foreground. In addition we see an
anti-correlation between the pressure and the fractional polarisation along the
S-shaped structure. Our results suggest that magnetic draping can be
effectively used to explain the observed polarised structures. This is likely
due to the combination of a relatively low magnetic field strength
($B_{eq}=1.21 \pm 0.12 \mu$G), enabling super-Alfv\'enic motion of the rising
lobes ($M_A=2.47-3.50$), and the coherency of the surrounding magnetic field.
Moreover, the draped layer tends to suppress any mixing of the material between
the radio lobes and the surrounding environment, but can enhance the mixing and
re-acceleration efficiencies inside the lobes, providing an explanation for the
average flat spectral index observed in the lobes.
