to the forum!
Well, if it's just the one rib that broke off, no big deal. If the break goes deep into the stem, that could be an issue.
If you go out and get lost in the central desert of Baja California, you will see cardón plants that have undergone incredible damage (up to and including repeated loss of the growth center) and grew out of it fine. I think the plant is naturally gifted at survival in your climate.
Avoid any sort of crack or crevice that may trap water, because I think the greatest risk is probably in the near term, related to rot. The plant will need time to heal but I would cut away the broken part to the extent that is reasonably possible. You want a clean, open surface that doesn't contain any pockets which might trap water. The lower the exposed surface area, the better. Ideally avoid cutting deep into the stem, but excising part of a rib isn't going to kill the plant and it probably would help at this point.
