Here is the original item:
Once class was over,
(A) Maria
used to meet her lab partner Chris at the diner near
campus (B)
until she discovered that Chris
preferred,
(C) to flirt
with the waitresses instead of discussing the chemistry homework.
You wanted to do this:
Once class was over, Maria used to meet her
lab partner Chris at the diner near campus,
until she discovered that Chris preferred,
to flirt with the waitresses instead of discussing the chemistry homework.
You do not need a comma after campus. Maria used to meet her lab partner Chris at the diner near campus is the main clause. Until she discovered ... is a subordinate clause. Generally, you do not use punctuation to connect a subordinate clause to the end of a main clause.
You might want to review the rules.