Much of this depends on how cooperative the neighbor is.
If the charger is a switcher, rather than looking into fixing a
switching supply, it maybe more cost effective to replace it with
an analog supply if it is simply a matter of applying the correct
voltage and current to an outside connector of the chair. This
assumes the battery charging complete condition is sensed
by something else internal to the wheelchair. (Defeating or
ignoring how the battery charge complete condition is sensed
could be dangerous.)
If the supply itself incorporates sensing for the battery charging complete
condition (which is common for Sealed Lead Acid Batteries) you
are probably limited to shielding the device or calling the manufacturer
for advice. (It could be defective). More likely, they have the standard
waiver that says it must accept and not create RFI and the owner is
responsible (which makes it hard on the little 'ol lady and makes you look
like an ogre). Shielding may involve placing the unit in another (metal,
grounded) chassis and running all leads through toroids. Beware of
restricting heat flow from the charger if placed in a shielded chassis.
Maybe it would be cheaper to buy her another brand of chair, trade it for the
noisy one, and resell, if possible, to a neighbor of your closest competitor.
Administrative requests: rfi-REQUEST@contesting.com