She's been feeding a line to Mitch.

Does the above mean "She's been making up a story to Mitch" or "She's been telling lies to Mitch?" Thanks.
It's a prepared story/statement of some sort -- something that's been concocted to achieve a certain effect. It could be an out-and-out lie, it could be intended to mislead but fall short of being an actual lie, it could be "spin", etc.
providing a certain canned/prepared text/reply to him
Teachers: We supply a list of EFL job vacancies
Yes, you're right. It'means to lie or exaggerate.
Students: We have free audio pronunciation exercises.