Most Helpful Guy
Whatever you do, in front of him be strong BUT do not act bitchy - that will actually come off as weak. You can cry about it at home.1
Most Helpful Girl
Remain very calm with a relaxed disposition then simply say, "Okay." then walk away. Even if you are not okay, don't say or do anything else. There are some situations in life where you must only speak on a need to know basis and say as little as possible despite whatever is going on inside. Getting dumped would be one of those situations.
Save the angry tears and long dialogues where you process the situation for your best friends and most trusted confidants. Don't share that with the one who is leaving you, turning their back on you, and either completely disposing of your presence or demoting it because that would be massively diminishing to your pride. You would look back on it later and be embarrassed. The best way to ensure that you handle this disappointing situation with grace considering how genuinely upsetting it is would be to say little to nothing in response then walk away. Have one of your closest friends on standby when you leave for you to cry to but do not cry in front of him or try to change his mind because it will only make it worse.1