Thank you for the clear pictures. Many tropical plants are used in flower arranging, but I can’t ID this one positively. It may be a species of hoya, such as Hoya obovata. If the plant begins to flower I may be able to tell you more. It may not be hardy outside over winter.
Your client could contact the florist in your area and ask what it is. The person who orders materials to make up arrangements may know it by name, or have a supplier’s catalog that will have it. Sorry I couldn’t be more specific.