I think an anonymous letter leaves too much room for him to gaslight her. FAR too much room.
I don't know how your situation went down, but I *DO* know that most BSs confront too soon (myself included) with insufficient hard evidence and get gaslighted to hell and back.
Can you follow him and get pictures of the two of them together? You could do that and go help her pull up phone records, credit cards, refer her here, etc. and create somewhat of a timeline before she confronts?
At SOME point, she's GOING to ask if you knew, you know...
So, I understand you're between a rock and a hard place. Your brother will be TICKED, but just let it roll off and help his W- and help him- salvage their lives. At some point, he'll realize you did it to save his life, both literally and figuratively, and he'll thank you for it. It's going to take some time, though.