I agree with adgal. I also want to add that even with a vasectomy, you should use condoms. Condoms are not just protection against unplanned pregnancy, they also protect again sexually transmitted infections, which a person can have without even being aware...and can pass on to a sexual partner.
3 weeks after a vasectomy you should be pretty safe. Yes, sometimes they fail and need to be redone, but not often. Secondly, with what sounds like ablation, combined with the fact that she is 45 and was probably on her period? I don't know the stats, but I am thinking close to 0. Anyone of these factors alone would make pregnancy unlikely, but combined? I don't think you have much to worry about.