I'll start by saying that I am not a doctor, physical therapist, or even a coach of any kind, but here's my line of thinking: If costochodritis ("an acute and often temporary inflammation of the costal cartilage, the structure that connects each rib to the sternum at the costosternal joint"-Google) is a reoccurring issue, there's a reason that it happens. If I say I have knee pain, we don't just accept that my knees were meant to hurt and try to train around it, we try to figure out if there's a dysfunctional movement pattern or physical deformity/injury that is causing damage, thus causing inflammation. Do I run poorly, squat poorly, did I damage my meniscus in a car wreck four years ago, was I born with the wrong knee structure for the sport I'm pursuing, etc. Since the ribs have to articulate during breathing or whenever the thoracic spine articulates, I would start there. Is there a breathing issue, such as constant coughing? Is there an issue with thoracic spine structure/function, or maybe an issue with the cervical spine, lumbar spine, or hips that forces the thoracic spine (and thus the ribs) to compensate?
As always, discomfort is probably alright, pain probably isn't. So do things that aren't painful, even if it's just tension generation drills or isometrics that cause no movement. If creating tension with the breath is painful, groove a lot of technique at light loads, so that the tension can later be applied to a sound movement pattern. I think the saying "First, do no harm" comes into play. Have the client do what they can, but until they are assessed by a doc and/or one of those FMS folks to figure out what structural/movement dysfunction is the root cause, I would guess (I'm doing a lot of guessing here) that progress would be difficult.
