In our game last night I was defending a player in the corner of the endzone... he then ran out of bounds for 3 roughly 3 seconds before re-entering the field of play and catching the disc near the sideline a couple feet from the endzone. I did not follow to mark him out of bounds and he was able to gain an advantage out of the play.. I could not find anything in the rulebook that says a player cannot intentionally leave the field of play... Can someone please confirm if this is legal or illegal to do this?

Totally legal, as long as he came back in bounds before he caught the disc. If he jumped from out of bounds, caught the disc in the air, and landed in bounds, that's a turnover (he's still considered an "out of bounds object" until he touches the playing field again, and the disc touching an out of bounds object results in a turnover). And of course he runs the risk of a thrower not realizing he's out of bounds and throwing it to him in the meantime.

hmm.. ok. thanks for the explanation.. still doesn't seem right that a player can just intentionally run out of bounds to create more space from a tight defensive mark...

I think it's a bit easier to mark someone who goes out of bounds because you know they have to come back inbounds eventually to catch the disc. Don't follow them out of bounds and just go up and down the sideline to adjust your position. You can also check where the disc is more often this way. If they throw your way then you should already be in a better position than the person you're defending.