Maybe just lay out your feelings and tell him that you understand why he'd feel uncomfortable but you're not giving up this friendship for his insecurities, and if he wants to be with you it's something he'll have to accept.

This is a sure-fire way to guarantee you eventually end up alone. Successful long term relationships are built on compromise and unfortunately sometimes those compromises mean cutting off old ties. She has to decide whether her attachment to her previous partners father is more important to her than her current partner or this will likely become a contributing factor to why the relationship ends in the future.

If this was an issue with her hanging out with her friends in general I would agree it's an insecurity but that is not what OP said and this makes her SO's worry a valid concern and not a matter or insecurity/controlling behaviour in my opinion.