Hanging or leaning on the chair is similar to hanging or leaning on a person sitting in any chair. It is often fine if you are friends, but inappropriate if you are strangers. Offer assistance if you wish, but do not insist. Talk directly to the person using the wheelchair, not to a third party. The person is not helpless or unable to talk.
People using wheelchairs use the same words.
Be alert to the existence of architectural barriers in your office and when selecting a restaurant, home, theatre or other facility, to which you want to visit with a person who uses a wheelchair. If conversation proceeds more than a few minutes and it is possible to do so, consider sitting down in order to share eye level. It is uncomfortable for a seated person to look straight up for a long period. Don't park your car in a parking place in an accessible parking place. These places are reserved out of necessity, not convenience.