Leave as soon as you make the decision that you're going to leave.
When I have employees try to give me a 2 week notice, I always tell them to "go now"
1 foot out the door is not "buying in" and unless you've bought in you're no good to the team
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assuming the person was not like "giving two weeks, but I really hate you/this place/everyone here", doesn't mean they are still not concientious about their position. maybe they simply have an opportunity they cannot pass up. maybe they are relocating for various reasons (and quite possibly need those two weeks of pay, since kind of hard to get a job if you ar ehonest it is only for two weeks). you would not want them to train new or existing employees on their functions?
OP did not say they did not like the team or HC, in fact seemed they were worried about leaving them in a lurch, just that they are not ready to move up when DD does not have to. should be a pretty amicable parting of ways really.
In fact, the one item possibly in favor of letting him know early, if you are on good terms with HC, they probably will understand, and might even assist in finding new opportunity for DD (we had this happen once, HC offered DD spot on roster, but 3rd on depth chart at primary position, and said they understood if we sought other options, and offered to help us. DD still takes lessons from this coach as well).