Debt sold to DCA after defective notice issued

[Me1964]

HI, new to this site and looking for some help please. My daughter had a default notice issued on a loan which was defective (demanded full amount and did not allow the 14 clear days, nor did the DN actually specify the date) Complained to FOS regarding this and unaffordability. Still with the Ombudsman, it has been escalated as did not agree with the adjudicator's decision regarding affordability. The adjudicator did state that the loan company had acted unfairly in issuing the default notice and they should remove it from her credit file. They have not removed it and my daughter has today received an email from a DCA advising they have now bought the debt. Could someone please advise me what action they can take being as they bought the debt without a valid DN notice being issued ? I have read some similar threads from many years back, particularly from TeaBoy if he is still around, but I am still unsure as to what action can be taken by the DCA as they bought a debt which had been defaulted and placed on her credit file, but the default notice was defective. Does the selling of the debt mean that only the arrears due at the date of the default can be collected ? (Have read this but it seems to conflict with other things I have read) The email from the DCA states the goodbye letter from the loan company can be accessed through a link i the email, I have told her not to access it. She has not heard from the original Loan company regarding the selling of the debt nor following instructions form the FOS to remove it. Any advice regarding how to respond to the DCA or advise regarding what they can actually do would be much appreciated. TIA