Meghan was also rude about Trump (said she'd leave the US if he became President) so not exactly blameless.
She wasn't alone.
She has the right as an American to hold opinions on Trump. I don't know why she would want to distance herself in any way from her remarks. Despite Trump's obvious preference for the alternative, America is still a country where citizens and lawful residents have the right to hold and express opinions.
For his part, Trump has the right to respond to her comments. He has the absolute right to end up looking like a petty, vindictive, orange dick who holds grudges.
And Harry also has the right to reply.
He could easily get a diplomatic cold. It's not unheard of.