Dragon Age: The Missing (Comic Miniseries)

It depends on how much the story feeds into DA:D. After all, Fenris was used in the last comic series even though he can be dead in some world states. However, the writers had the survivors heading down south at the end, apart from Tessa and Marius, so their story wasn't directly feeding into DA:D but Tevinter Nights and the Dread Wolf Take You Mortalitasi's tale. Since the blurb suggests there is a direct feed into DA:D with this latest series, it seems unlikely they will use someone like Anders unless he is not going to survive it. That would create problems of its own, since those people who have Anders alive in their world states will not be happy if he dies off screen. However, I do agree that he best fits the "former" friend tag since Varric did not have anything complementary to say about him in DAI, so I definitely feel Varric no longer considers him a "current" friend.

It is possible that one of the Grey Wardens from Tevinter Nights could fit the bill if you think about their life prior to joining the Wardens. In particular I am thinking of Evka. Could she have known, say, Harding prior to joining the Wardens? Or had some sort of connection with the Inquisition. According to her history in Hunger, she had been a Warden for around 3 years prior to that story, so if that was set around 9:45, that would bring her time outside the Wardens into the period when the Inquisition was operational. So, could Harding have come into contact with her when scouting out the Deep Roads for the Descent? If the person is a former friend of both Varric and Harding that would point to an Inquisition connection.

Another possibility is one of the Inquisition agents from the War Table missions and multi-player. In particular I am thinking of Korbin. He was a member of the Legion of the Dead before joining the Inquisition and WoT2 says he was sentenced to the Legion when he took responsibility for someone else's crime. I can imagine both Varric and Harding having time for such a selfless individual, probably socialising in the tavern at Skyhold. When the Inquisition was disbanded/cut down in size, presumably he went back to the Legion and they work closely with the Grey Wardens. So, he would qualify as a former friend and have a reason to be in the Deep Roads, plus a connection with the Grey Wardens.