By the time, this event is triggerred, data exists in po_headers_interface table, but deleted after the same.

So, in your custom logic being invoked from the subscription of this event, you can use some thing like this to get the RFQ data

select po_header_id into l_po_header_id

from po_headers_all

where po_header_id in (select po_header_id from po_headers_interface where interface_header_id = p_event.getValueForParameter('Interface_Header_ID'));

Once you get the po_header_id, you can get corresponding RFQ data from po_headers_all.

After this subscription is fired, data no longed exists in Headers interface table. SO, you should capture the same in custom logic being invoked from subscription.