A lot of us feed green beans, either frozen or canned, as a treat or filler if they are dieting. I could be wrong but I haven't seen anything about kidney beans.
Welcome! Wonderful of you to rescue!
In the meantime feel free to post pics of Lunchbox. I was about 11 when i first met a bullmastiff. Fell in love in an instant and I'm sure it's what led to my love for bulldogs.