I believe that it can be dormant for that long... thats why noone ever knows in a relationship who had it first... or if anyone cheated and whatnot.
I got hpv sometime in college and did not know it until i was married with kids. I had a reoccurance 13 years later. there is absolutely no way to know when you got it and reoccurance can happen with or without symptoms.
As a women and a Mom, may i also say...you should not have to worry about this for your mom, she is a grown up and should be seeking counseling right now to help her cope with her doubts.
Good luck.
I'm not a doctor too, but I'm also in complete agreement with simplynat
There is no possible way to know for sure when or from whom a person contracted HPV. It can absolutely be dormant for over 20 years, most will never know they have it because usually there are no symptoms. HPV is fought by the immune system and in times of stress, pregnancy, ill health - it can be brought out into an active infection. Also, a woman can test negative for HPV but still in fact have it. The digiene test can only detect whether there is an active infection at the time (if it is dormant it will not be detected) and if there is an active infection whether it is of the high strain or low strain (wart) category. Unfortunately, in this situation, your mom cannot tell from HPV whether he was faithful or not. Your mother can only go from her heart.
Also, I am not a medical professional.