Hi Christophe
You asked "when can the forward bend be practiced and when should it be totally avoided?" I would say, if the student is flexion intolerant (which means flexing the spine creates pain), tbey should not do forward folds, unless their doctor/therapist can tell them just how much they can/can't do. People with discogenic sciatica would be in this camp and probably should avoid forward folding, but the challenge is -- what degree of forward folding must they avoid?
If the student doesn't feel any current pain, are they are risk? Yes, of course--there is always a risk, but this doesn't mean the risk is big. As in all things therapeutic, everyone is different. Each student has to work out her limits along the antifragility curve (see this
article). Too much is bad, but so is too little. How much is too much? The student will have to listen to her health care team and experiment to determine the answer to that.
Sorry that I can't give a black and white answer, but people are too variable for black and white to apply.
Cheers
Bernie