Can you 离开 a person? (as in physically separate from them?)
Yes, but I probably would use 分开 in your example if I wanted to emphasise the fact that we separated (and that it was a mutual act). For instance:

我们在那个地铁站分开，因为他要去医院而我要去百货商店。
We separated at the train station, because he needed to go to the hospital, but I needed to go to the department store.

But saying 离开某人 can also work. For instance, when the separation is more &quot;one-sided&quot; (in terms of how it's described), it makes sense to use 离开. For example:

我离开我妈妈之后，我去了加拿大读研究生院。
After I left my mother, I went to Canada to attend graduate school.

At the very least, this usage makes sense to me as a semi-native speaker of Chinese. Notice that the connotations of 离开 and 分开 are somewhat different. Moreover, 分开 wouldn't be used with an object to state A left B - rather, it would be used to state A and B separated.
I could also use 离开 in the original setting as well, but mostly in situations similar to the one I described above. For instance:

在地铁站离开他之后，我去了百货商店。
我在那个地铁站离开我朋友，因为我要去厕所。

But 我离开他 (as a sentence in itself) sounds odd without any appropriate qualifications to the statement. Moreover, using 离开 to state something like 我在地铁站离开他，因为我们坐不同的地铁线 sounds odd to me (because the subordinate clause focuses on 我们）.