Does not "one-on-one" mean one person versus one person? Yes, it does. But your OP doesn't say that the client will always assign a single person to interact with your company's representative.
One of the former students. "One of" refers to a group. The group that follows is plural. "Students" is plural of "student." Consider the statement, "one of the team." A team is a group. It can be referred to as singular or plural, depending on the context. In this case, the sentence refers to a larger entity which "one" is part of. "Students" in the instance you refer to is the larger entity.
How one and one's is different from other indefinite pronouns The possessive of one (one's) is formed the same way as the possessive of other indefinite pronouns, such as someone (someone's), but it is used a bit differently. For most people, one is consistently used with the possessive form one's.
Which one is grammatically correct or better? I have two assignments, One of them is done. I have two assignments, One of which is done. I watched a video tutorial that the teacher said the ...
pronouns - "One of them" vs. "One of which" - English Language Learners ...
The "one" could imply that of the alternates only ONE choice is possible, or permitted. "Which" alone could indicate several choices from the set of alterates could be selected in various combinations. Of course, speakers are often very imprecise about their meanings & intentions when saying "which" or "which one". Usually the context makes it clear if the choices are mutually exclusive or not ...