I, for the most part, agree with this.
Obviously, you have all the right intentions, but relationships are weird things. Your boss's explanation of the credit card charges is so incredibly lame that I'm sure your friend, somewhere in her mind, has an idea what's happening. She may have a powerful reason for repressing these thoughts. If you challenge her on this, she may interpret it as an aggressive act on your part.
I know you want to look out for your friend, but you have to be careful. There may be ways to give subtle hints that you do not trust this guy, but I think it would be a mistake to take a hard line and get yourself in the middle. It's likely that she will have to work it out in her own way.