You can't assume you would fit in at the company event for her. You would have been invited had you been wanted/needed there. Being there changes the dynamic totally. Since we don't know all the facts, I think you need to do something special just with her to honor her. Sending flowers to the workplace can't hurt or offend anyone and is a good idea. But do something with her or for her that she will enjoy. That could even include leaving her alone.