Buying out the IGG pile and then grabbing as many duchies/provinces as you can is a surprisingly strong strategy in the absence of trashing and other cursers. I suspect I'd play this board similarly to your opponent, though he did make a few mistakes. The monument/silver opening was good but I don't know why he bought the spy or the potion. Theoretically, the philo stone would be strong in this type of deck, but I'm not sure its worth the opportunity cost. You waste 2 turns getting it into your deck. Also late game, he should've ignored harems and gone for duchies. The +2 coin is not as helpful as the extra point.
As a defense, well option 1 is to just go for the IGG's yourself and try to beat him on that front. Option 2 is to get an early embargo and embargo the pile of them early, then play a big money game. Trying to use highways to lower the cost of provinces to smuggle was probably a losing proposition even without the curses in your deck. Highway is not strong on this board.