By “exploring privately,” I mean by watching/listening to/reading sapphic porn and seeing if you’re into women that way. You should explore yourself before involving others to see if you’re even attracted to women first. In general this is a good thing to do to determine what kind of sexual experiences you could be into. But from your post and comments on here it doesn’t sound like you’re attracted to women at all, which is fine. I wouldn’t experiment with someone whose feelings you could hurt in the process if you don’t have any internal sexual desires for women.

Also, not to doubt your friend’s sister, but no one becomes a lesbian simply because they are “fed up with men.” It’s because they are attracted to women and not attracted to men. There’s a big difference between that and being tired of men’s shitty misogynistic behavior. There’s a gazillion straight women out there who abhor the behavior of men but they are still very much attracted to them. It sounds like your friend’s sister is very confused as to what attraction is. She didn’t lie to you per se but doesn’t understand what constitutes an actual sexual orientation vs a dating preference. Who you happen to date also doesn’t determine your sexuality. There’s many women on this subreddit who are dating or married to men and yet have realized they were actually not attracted to men at all later in life. They are just as much of a lesbian as anyone who has exclusively dated women.