The royal dating pool has historically been fairly limited — princes and princesses mostly date within their social circle and often remain friends afterward, so it's inevitable that exes will end up at most royal weddings. Diana was devastated that Camilla Parker-Bowles was a guest at her wedding, but there were no qualms when William and Kate invited six of their combined exes to their own big day.
It has been reported that Harry may have invited his most significant ex Chelsy Davy to his wedding, and he will almost certainly run into his other significant former flame Cressida Bonas at his cousin Princess Eugenie's wedding, as the pair are best friends.