As long as you use the library by yourselves and only one/two computers at a period of time, I believe it's legal, period.
Hypothetically, I can have a client who has very low knowledge about composition (in which I will create a composition or whatever midi files) or a client who are very deep knowledge about composition, who gives feedback/review, some parts of composition or initial midi files to render the sound, so he/she can concentrates on other parts that he/she very strong at. is it forbidden to do this?.... I don't think so, If it is forbidden, the EULA or whatever contractual agreement will also say about what types of clients you can take. I never see this kind of EULA.