Charizard OHKO'd by Dig?

So it's been a long time since I have played much Pokemon, to be fair, and I just got into this romhack, which is utterly fantastic btw. I'm playing the vanilla version, not EX, mostly just to keep things simpler and because the first three generations are the ones I played with as a kid.

I got to the fourth gym for the first time and sent out my Charizard vs. Giovanni's Sandslash. He used Dig, so I used Roost because I figured it shouldn't hurt me, but then of course my level 45 Charizard was annihilated in a single hit and I almost wept. Aren't flying types immune to ground moves? Is there any way to check that he has an item or ability to ignore that? I guess I'm mostly still grieving because I totally could have survived that if I knew it was coming and sent out someone else.