I praised my friend's novel after reading only three chapters. Should I finish it before admitting that?

My friend asked whether the ending worked, and I said, 'It stayed with me.' I had not reached the ending.

She'd sent me her first completed novel six weeks earlier. I read three chapters, liked a scene in a railway café, and kept intending to return to it. When she asked if I'd finished, I said yes because I didn't want to disappoint her. She looked so relieved that I followed the lie with another compliment.

Now she wants to meet on Saturday with a list of questions. One is whether she should cut a character I've never encountered. She isn't asking me to promote the book or give a professional opinion. She wants a friend who has actually read it.

I could probably finish before Saturday. Part of me thinks that would make the conversation useful and spare her the humiliation of learning her proudest moment got a fake response. Another part knows I'd be using the deadline to make the lie disappear.

Should I tell her before reading the rest, or read it and then explain exactly what happened? I don't want my apology to become a request for her to tell me the book wasn't worth the effort anyway.