Where are you located and what lodge?
As you have already heard here, a lot of what you hear out there is a bunch of hog wash. I generally feel sorry for the innocent folks that speak the anti-Masonic rhetoric, because they have been wildly misguided and misinformed by someone with ulterior motives. I would say that in most cases these fall into two groups 1) fellows that have in someway become dissatisfied with their lodge experience or 2) folks wanting to sell books.
You will likely begin to notice, at some point in the future, the we, as brothers, encourage each other not to argue with those that are misinformed. To do so is usually futile and is more likely to have ill effects on your reputation that positive effects on the craft. I had a brother tell me once that he thought he should defend Masonry. I asked if he really thought that a something with a history as strong as Freemasonry needed defending. Opposition is nothing new to Freemasonry, the first anti-Masonic books hit the streets within a couple of decades from the first Grand Lodge meeting in 1717. I believe that opposition has at times and in various ways been a benefit to Masonry.
If the people you here saying these things knew your FIL, I would simply ask them if they think that he would ever be involved with something so sinister or "of the devil". That is a response I use quite often, because most folks I know have someone that they place in high regard that is or was a Mason.