Trump should point out the facts, that he was invited well before the “shutdown” and that her antics are wholly political.
If you read Trump's most recent letter, he says he was invited AFTER the start of the shutdown, on January 3, 2019. He accepted that invitation. I recall seeing one of Trump's tweets where he even made mention that there was a contract made (he was referring to the letter of the law contracts - an offer and an acceptance = contract). She has now broken that contract.