Do you mean it's difficult (e.g., takes muscle power) to steer?
I have an '83 240D and the steering is extremely easy.  No stiffness
whatsoever.  It will turn on a dime.  In fact, I was considering having
my mechanic check it out because it feels a little loose to me.  I've
never heard of this problem before.   Perhaps your mechanic can
make some minor adjustment?   All the best.

Helen

Steering effort can be changed but first one must know that the steering
mechanism is OK. Is the power steering working as it should? Are the
ball joints OK or worn, same with the idler arm and tie rods. There's a
steering damper whose function is to stiffen the small steering
movements and so absorb and prevent road vibration from being
transmitted to the steering wheel. Any competent mechanic can check
these items.

Then there's the front wheels' alignment which consists of the wheels'
camber angle to the pavement 90 degrees less a fraction of a degree,
their toe-in or parallelism to each other - about 1/8" closer together
in the front than their trailing side and finally, the caster angle
which is the difference between the wheel's top vs. lower pivot point.
Think of a door on hinges, the lower hinge being NOT vertically (plumb
line) below the upper. So?
The caster angle is the effort she's turning against. The other side of
that coin is that the caster angle helps to self center the car's
steering to the straight ahead position. There are specifications for
all these settings and those have a small range of allowable values:
i.e. 10 degrees +/- 30' (that's 9.5 to 10.5 degrees). I suggest that if
all the mechanical aspects are checked and found to be in order that the
front wheels be aligned with a bias toward REDUCING the caster angle to
its minimum allowable specification.

I found the steering of my '80 300SD to be needlessly heavy and that did
the trick.

You should also know that under inflated tires cause heavy steering, try
28 to 30 psi in the front and 30 to 32 psi in the rear - that might
sufficiently ease the effort to avoid the alignment adjustments.

If none of these solve the steering effort problem she should test drive
a Honda, I believe these have very light low speed steering effort.