> I met a schoolmate on Google. She was a graduate of ABC College in 2000.

Perfectly fine. We could apply a false pedantry and say that then she was also a graduate of ABC College in 2001 as well, but that is the sort of false pedantry that is apt for the season, because it belongs only in the sort of riddles that come in Christmas crackers. When we say someone was something in a particular year, it's implied that they weren't before. Whether they were or were not after that depends on the nature of what we have said they were, not on riddling.

In all, perfectly fine and perfectly clear.

> I met a schoolmate on Google. She was a 2000 graduate of ABC College.

The form "A [year] [noun]" isn't widely used, but graduation is one of the cases where it is used, meaning that they graduated in that year. So again this is perfectly fine and perfectly clear.

You could also say:

> She graduated from ABC College in 2000.

This gains nothing in correctness or precision, but I prefer this simply because in the context of meeting an old schoolmate I am more interested in learning what they've done in the meantime (graduated) than in their qualification and educational status (being a graduate). This would be different if I followed with something that made that status relevant ("… I suggested she send her CV to our HR department.")

It's also technically correct to say:

> In 2000 she graduated from ABC College.

But this puts a bit more emphasis on "2000" than I think it warrants, unless perhaps one was listing a timeline of events.