You ask if you should “stay out of this,” but the reality is that you are in this. Both you and your husband-to-be bring certain patterns to your marriage, some of which will be gifts and others of which will be challenges for each of you. Your fiance’s anger with his father could be one of those challenges, if the anger is intense and consistent. We all get angry and so my point is to discern if his pattern is something in need of change. If so, his forgiving his father would actually help his and your relationship.
I recommend that you do some pre-marriage work in which each of you explores some of the unhealthy patterns which you have learned when growing up. Each of you should forgive and help the other in the forgiving. In this way, you are not singling out your fiance as having some kind of unique problem. Your working on this together could strengthen your bond.
The issues of marital partners forgiving people from their family-of-origin are discussed in the book,??The Forgiving Life.