I think most bed manufacturers recommend that people do not sit on the edge of beds on a regular basis but I think this is a totally unrealistic caveat to put on to a condition of use for a bed.
Did the bed come with a warranty of some kind? I'd be tempted to go back to them, telling them that the bed was not fit for purpose and ask for either a replacement or a refund. If they are still unco-operative you have the option of taking your case to the small claims court, Before you do this I would suggest you contact the Trading Standards Department of your local council. They may be able to offer you some advice on how best to proceed with your complaint.