I am unsure of rules in the UK but where I am treatment is discussed only between the patient and doctor.

I am guessing that, because their medication is being injected, that there already are compliance issues.

Unfortunately (here in Canada anyway) the law cannot compel someone to take their medication.

What can you do then? You can be supportive and encouraging. You can try to help your loved one establish a routine. Will you be in a position to hold them accountable to it? These are things their mental health care team may be able to help you with.