Actually, I don't believe that. This is not, and never has been, part of Catholic marriage law, from the beginning until now. In some cases, the Church actually encourages it (1 Corinthians 7:8-9; 1 Timothy 5:14 ) and some of our canonized saints were remarried widowers (e.g. St. Thomas More, Blessed Adrian Fortescue, Blessed Humbert of Savoy who was widowed and remarried several times, etc.).
Any Catholic said remarriage of widows or widowers would be an outstanding ignoramus. Are they FReepers? Would you like to name names? I'll go after them myself.
About people spouting off inaccurately about Catholic doctrine: might you ask them to check the Catechism? Might you take that advice yourself? Hmm?
I did even better....I got a lawyer!
http://canonlawmadeeasy.com/2017/05/04/ex-spouse-dies-second-valid/