If your dad is legally incompetent or his ladyfriend could be guilty of financial abuse, there might be legal steps you could take to protect his assets. But it doesn't
sound as if wither of these are true.
As you know best, your father was likely the independent type, not used to taking advice from his kids or maybe anyone else, even before your mother died.
It sounds as if you've already tried to have a heart-to-heart with him about his spending and what it might mean for him and for you and your sister. You might also try having him meet with a financial advisor on the offchance he would be willing to do so. Oftentimes, people outside the family have an easier time breaking through to those who won't listen to insiders. And this can be especially true if the outside adviser is an expert of some sort, who can give wise advice without appear to "take sides."
That urged, the reality is that there may be nothing you can do to change you dad's unpredictable spending right now. You should also know that many people who lose their spouses go through a reckless period for a while, in an attempt to find and assert their new independence.
Tough love advocates would tell you not to keep intervening and saving your dad's credit, as tough as that may be. In the longer run, it may only act to sanction his irresponsible behavior as there would seem to be no negative repercussions from it.