Thanks for all the suggestions/comments/experiences. They are all most useful. Some I can't follow through, as (I think) they require a more advanced Anc sub than my friend has, and money is tight.
One question (and sorry if this is naive). In the Anc matches, I have filtered on just Paternal. Within that, when I look at each matches "Shared Matches", they seem fall in to two distinct groups that share (seemingly randomly) with others within that group, but not with the matches in the other group.
Is it a reasonable assumption that these two sub groups of the paternal matches represent my friend's paternal grandfather and paternal grandmother relations?
And further, if I work up a couple of trees for each side, they "should" (could) vector in at my friend's mysterious father?
Lastly (and more of an opinion thing)...
Why do people get tests done, appear as matches, only to not have trees and not have a user name that gives any clues how they might tie in? Not having a go in any way, just curious why.
Question?
Are you OK creating and populating a spreadsheet?
If yes, then look up The Leeds Method.
Create a spreadsheet that replicates what you see in the method and populate it with the DNA matches, including the shared matches.
It should be pretty clear as to which DNA matches are the ones to concentrate further research on.
Lastly, that is the £10,000 question.
A few reasons why:-
A lot of DNA kits are gifts and the recipient is only interested in the Admixture results.
Privacy, for whatever reason the DNA results are not wanted to be effectively shared.
The DNA test and subsequent tree build is more involved than they first thought.
People searching for unknown Parent(s) or Grandparent(s).
Realisation that using DNA results is more involved than they thought.
Odd user name like Fred1234 mean nothing and it may be that they want to keep the results to themselves due to family objections (we have had this posted and I also have a relation who is against my DNA being out there).