Retirement parties usually involve the participation of the co-workers. In advance you'll want to contact co-workers, maybe have a "key" person helping you, to have them come up with funny or memorable stories involving your mom, and how having your mom as a coworker helped them. Little anecdotes.

The party should be like a cocktail reception with appetizers, and set aside a time for the coworkers to tell their stories, mom's manager to speak, maybe the owner of the company to speak, and to toast the retiree.

Also put together a scrapbook with work memorabilia, pictures of coworkers, pictures from previous company parties, etc, as a keepsake.