We do not know the situation. She and Tom clearly clicked at the Met Gala where they were co-chairs. Calvin Harris had been distant and looked unhappy with Taylor for awhile. Tom was active with her, danced with her whereas Calvin Harris refused to even attend WITH her, any events really. There are no children involved and no marriages so I say there is nothing wrong with it. Let her have fun, people are going to always joke about her dating life and songwriting anyway so she may as well have fun in the process. Now the jokes can stop about Taylor being dumped, as it appears she might have dumped Calvin. Rightfully so, he look miserable with her and that had to be embarrassing for her.
Did she fire Calvin as her boyfriend and set up a contract with Tom to be her new hire? He is in the running to be the next Bond, something high profile like this could push him to the front of the line to snatch the role, and Taylor would have a A-list movie star on her arm as she pushes new music. Sounds like a win for her and him.