Absolutely nothing important happens in "A Christmas Carol." Amy and Rory aren't even a part of the main story. You can skip right to Season 6.
Well, then he could stop watching/reading fiction altogether... he would never miss anything of any importance.
Well, you've clearly stopped reading, or you would understand why I responded the way I did. The OP specifically said he didn't want to miss out on anything important before starting Season 6. Since nothing in A Christmas Carol
has any effect on Season 6 in anyway, he is safe to skip it without missing out on any important plot elements. I didn't say "Never watch it."