Your friend's Solicitor will have a copy of a standard form called "Fixtures Fittings and Contents" - he should have given him a copy. This is a comprehensive list of most items in a property and has three columns: Included in the sale; excluded from the sale; none at the property.
Under light fittings it says: "if these are removed, it is assumed that they will be replaced by ceiling rose, flex, bulb holder and bulb."
Loft ladders are not shown on the standard list but there is a space to add extra items. I would have thought that the vendor was under a duty to list the loft ladder and say if it was included in or excluded from the sale.
Did the Estate Agent's particulars mention the loft ladder? If it did, it should have been left.
Your friends should speak to his Solicitor - loft ladders are not cheap.