She could use euphemism it is a figurative language that is used in place that can suggest something unpleasant but in the kindest way possible.
Paying attencion,Leaning in, and eye contact
Hello his name,
I am very sorry but I am unable to join you for dinner tonight. However, I would still very much like to go to dinner with you. Would we be able to meet for dinner tomorrow?
Thank you,
Your name
Hopefully that’s not too formal haha
