Cast iron welding requires preheat, interpass heat, and postheat management to a surprisingly exacting degree. You should also typically use special rod with a high nickel content, which they probably didn't do either, so now the surface is all contaminated down to whatever the puddle depth was, and the crack is contaminated all the way through.

In my opinion, this collar will no longer be trustworthy, and it would be a good idea to replace it if this drill press will see any heavy use at all.

If you're just using it on wood or for drilling holes in relatively soft metals, it should be fine if you get it brazed and have the bore cleaned out by a machine shop. If you're going to use it on steel, get the platform replaced.

And make sure to tell your friend he's an asshole.