Often your thought process is more important in these grey-area questions.
First off, putting in time to help others make a big difference, and the mentality of everyone pitching in is really important for morale. Those bonds are really important; who knows when you’ll need a trainees help in the future. However, it’s also important to note that two trainees aren’t necessarily better than one. If you haven’t been briefed on the deal, it could take more time to bring you up to speed. It also depends on the work that’s being done/when it’s due etc. So I’d want to ask the trainee some questions – what kind of work are they doing? If the trainee was printing/photocopying for a meeting tomorrow, I wouldn’t necessarily drop my plans.