I agree that the idea of a 54-year-old man dating a 16-year-old girl is certainly cause for concern, at least in the Western world. Assuming their courtship was chaste though, Church law allows girls to marry as young as 14 (canon 1083 §1, Code of Canon Law) – although episcopal conferences can set a higher age more suitable to their own culture (canon 1083 §2) – and so the couple technically would have done nothing wrong, presuming that the girl’s parents or guardians agreed to the relationship.
As for their marriage, twenty-two years would mean that this woman is a legal adult, both by the law of the Church and by the law of many countries. Assuming that there were no impediments to the marriage, as you have confirmed that there were not on your father-in-law’s side, the Church would have had no problem marrying this couple.
While I sympathize with your concern, I can only recommend that you do your best to maintain a good relationship with your father-in-law and treat his wife with courtesy. Doing so is not only the right thing to do, but it will give you and your spouse the opportunity to keep an eye on the situation and to help your father-in-law should the need arise.
