I agree that residents should not, in general, store personal property on the common property. Able-bodied people can easily move their bicycles or prams and should not leave these in the foyer. On the other hand, there are times when it is appropriate to not apply such rules pedantically.
An elderly person’s life might be made much easier if a blind eye is turned to leaving their walking frame in the foyer. I would reject ‘thin edge of the wedge’ arguments. If the elderly person is made to try to get in and out of their front door while also bringing in the walking frame, it could make a big difference to their health and general well-being. It could make the difference between whether they get up and move about outside or stay indoors all day.
If it is a trip hazard in one particular spot, have a quiet word about putting it somewhere slightly different.
As for “there is a history of this particular owner complaining to and about other residents over various perceived issues”, this is irrelevant. The resident might be thoroughly unpleasant in other, unrelated respects but that should not matter. Treat each issue separately.