I would say use her birthstone, but for April, that's a diamond
Cloud and Bruce have it. Are you sure she would actually want a ring? Originally it was a symbol of possession, some women (myself included) are not so keen on the idea of being possessed....
Just ask her first. If she's all you believe she is, her answer and "the moment" will not be based on the production of the right ring at the right moment.
Also, if it were me, I would not want your friend accompanying us. If you truly need a picture, have your friend on standby and call them once she has said yes and agrees to your friend being "the first to know" and to the picture.