I’m so sorry your mum’s having such a rough time of it. The first thing to note though is the positive news that she has no cell changes - that’s great!
The second thing is that your dad’s affair is obviously a difficult thing emotionally, but your mum needs to completely separate that in her mind from this HPV diagnosis as there is no possible way to know if it is even a factor.
HPV isn’t any kind of indicator as to fidelity or sexual behaviour other than to note that, yes, someone with HPV is likely to have had sexual contact at some point in their life. But short of those who are too young, or those who have sworn to abstain, I think it’s safe to say that’s expected of us all at some point! HPV is incredibly common - so much so that you can realistically assume every sexually active adult will have it at some point. Many will never know as they’ll have no symptoms of anything associated with it, many will clear it from their body before it has reason to be detected by for example a smear test, and many of those who are found to have it will also clear it without issue.
It can lay dormant for many years, so the reality is that your mum could have gotten it the first time she ever had sexual contact with anyone and it’s just reared its head now. There’s no point in wondering if she got it from your dad as a result of infidelity. It’s possible, but so are many other scenarios and there is zero way to know.
It also isn’t something to worry about in itself. I know this seems an easy thing to say - I think it’s natural to be thrown by it. I thought I knew all the facts and still had a wobble over the HPV bit (I had abnormal cells too). My GP and my consultant told me kindly, but very firmly to put HPV out of my head and that all I needed to worry about was staying engaged with the screening programme designed to keep people safe.
It’s not “dirty”, it doesn’t mean anyone has done anything “wrong” - it’s unfortunate that there is the issue of your dad’s affair, but I really would try to see these as very separate situations. I hope your mum can find some peace with this.