so my question to the FI is what do you brief to your student when they go on their solo navigation. to which altitude could they go down if the weather was going down and they need to divert.
The school should have established a minimum altitude for training; you do not authorise solo flight if the conditions are such that they would need to go any lower. This is what solo authorisation is all about, so you brief them that they do not go below the authorised minimum. It will mean that there are some days when its not suitable for solo even when the weather looks quite reasonable but, the instructor has a duty of care to ensure that the student is not placed in the situation you describe. In the very worst case, they will have been taught how to conduct a precautionary landing.