Hold your horses just a moment.
Remember that this anonymous person will not be acting for your benefit - they will be trying to benefit themselves or attack your df or both.
Assuming df=fiancé - is there a wedding date set and soon?
how he fell for her before but stepped away as he is in a relationship!
Could it be that he is actually now doing the right thing and cutting contact with this woman because he loves you and doesn't want to jeopardise your relationship - and she is doing this deliberately to split you up so that his decision to choose you gets comprehensively trashed.
Remember that screen shots are easy to edit - they are just pixels in an image file. She could have picked out all the juiciest bits (from an admittedly very inappropriate emotional affair) and chosen not to include anything the poison-pen person didn't want to.
My recommendation is that you make arrangements to be completely alone together one of the days of the weekend with nothing else happening so you and df can have a full and honest talk about this.
Possible starting line something like "I've been sent some Facebook screenshots that show that our relationship isn't what I thought. This is your one and only chance to come clean and tell me everything that has been going on, then maybe we can work out a plan to save our relationship."
Some dear friends of ours (godparents of our eldest) had a "blip" six months before they were due to get married. He was excessively stupid and let his dick do his decision making. The wedding was called off but he was shocked into realising that his df was his real love, the ow was a foolish weakness, and he rededicated himself wholeheartedly to monogamy henceforth, and she forgave him and they rearranged the wedding for a year later - they have now been married 12 years and continue to be happy and (as far as I know) faithful.
That which has true value is never truly lost. It can be re-found, if wholeheartedly sought.