Schwinn makes an adult helmet with a magnetic type clasp that is easy to open/close. The helmet model is called the Adult Form + Fit EZ Magnetic Snap Buckle. I found it at a local Sears store, but since Schwinn is such a common brand I'm sure other stores may carry it as well.
Someone who has an issue with hand strength can operate this helmet buckle. I tinkered with it and it seems to work quite well. ...thought I'd pass on this information to anyone who might be interested.