It very much sounds as if your dad's now ex partner is not "absolutely lovely" and, while your intention of continuing "to be her close friend" is commendable, I would caution you against becoming over involved with a woman who has clearly more than one side to her personality.
It also appears that your dad has not fallen victim to a clairvoyant who preys on "vulnerable men", nor has he taken anything other than a pragmatic view of his attraction towards another woman.
Fwiw, there is a school of thought that has it that at some point in the soul's journey it is split into two giving each of us a twin and that we are individually part of a group of souls who work together to further our own and each other's attainment through the spiritual and earthly planes of development/learning.
This means that we each have a 'twin flame' and a number of 'soulmates' to whom we are more attuned than with others outside of our group. However, this doesn't mean that we are destined to meet our twin or any of our soulmates during this or any other lifetime as, while some may choose to incarnate at the same time or in the same earthly family/social circle in order to further their progress, others will choose to incarnate at other times and in circumstances where it's highly unlikely their paths will cross with their twins or other group members.
It's a belief system that is as valid as many others, and its one which has become increasingly popular with those who do not find anything to be gained from man made religions but, neverthless, are not devoid of any sense of spirituality about 'being' who and what we are.
Overall, it would seem there's some lessons for you to learn about not rushing to judgement and to trust that depth and breadth of your dad's cumulative experiences will continue to enable him to exercise sound judgement about how he wishes to conduct his life.