Can this cause a delamination, or worse a leak? The upside to this is I will be installing an awning right over it, so cosmetically, it'll be hidden. Should I bother getting it repaired or just fill it in with some Sikaflex and put the awning over it?
It can certainly cause a leak and potentially some delam. It needs to be repaired/sealed. Some caulking (Sikaflex) will do it or you can get cosmetic with it and do a complete fibre glass repair which is much more involved.
Sad to see...hopefully you neighbor makes some amends in some way.