If your consultant is an Advocate or a Lawyer, then, even if they don't use the prescribed format of invoice/bill under Rule 4A of the Service Tax Rules, 1994, then there is no harm in it. Since, they are not liable to pay the service tax. The service recipient is required to pay the service tax under reverse charge mechanism. The service recipient can avail the CENVAT credit on the challan through which they paid service tax under RCM.
Further, if your consultant do not come under the category of Advocate/Lawyer, then, he has to adherence with the compliance of service tax rules to issue an invoice/bill in prescribed format or having the prescribed ingredients.
We also endorse the view of Respected Kasturi Sir.