So it’s like Romeo and Juliet, only it’s Juliet with Romeo’s grandfather.
It could be worse. I saw an account of a woman who divorced her husband and he went on to marry her mother! While that sort of thing could cause problems for them, I wouldn’t be affected.
The only test for any marriage, regardless of age, race or religion, is love. And it should be administered by the bride and groom - not the parents or a gathering of lawyers or a bunch of bored strangers sitting at keyboards.