I have an Oakworks and my husband, who is a very big guy, is easily held by it. It's not a very adjustable chair, though, so it's not super-comfy for him.
I agree w/EM -- just let her try it out, see it she likes it! You can suggest feet flat on the floor, but I would do it only as an option, not the "fat" option. Here's a thing to know: a lot of bigger folks silently and constantly assess furniture, wherever they go -- restaurants, friends' homes, a new workplace. There is very little more upsetting than breaking or getting stuck in the furniture, and people know how to avoid doing it! So just as she knows she is big and doesn't need anyone to tell her (not that you have), if she had the slightest doubt about the chair holding her, she probably would never have brought it up.
I have generally found that what the table will hold, the chair will hold. Just a matter of positioning.