If you feel like you are working much harder than your friend and still losing, this is an immediate alert that you should just be changing how you are playing and how you are approaching the game. Winning is about playing smarter, not about playing harder. Also, throw whatever ideas you have about being better than your friend out the window whenever you play him, none of this "I'd beat way better players than he could" matters in the head 2 head, and this is true for literally everyone to have ever played the game.
Getting good against Sheik is a lot of getting used to her timings/habits. Ftilt has great startup, but enough cooldown lag that you can grab her if you bait it. If you are really confident, you can try to bait it and overshoot it with dair. Utilt can be shieldgrabbed between the first hit and 2nd hit, but after the 2nd hit Sheik is safe for another action.
Fair and Nair can AC into her tilts/jab, so these moves are actually safe on shield.
All of her moves are not great at lower %, and don't really start leading into massive combos until past 40-50%. Crouch cancel makes this MU way harder for Sheik.
Once you've established that Sheik's moves are not great at lower %, she relies on grab heavily to get her through early game.
What Falcon does: Nair is a poke move in this MU. Space it well always cuz it can get CC'd grabbed if misspaced, but you can use it to bait out Sheik's options (ftilt especially). Use nair just to feel out Sheik, don't need to commit to a followup unless it bounces sheik up or if you've conditioned sheik properly. Bait out rolls/ftilts, grab them. Dair is like a higher-reward, higher-risk grab, and straight up destroys crouch cancel if sheik has started getting frisky with her crouches.
Sheik is in an awful spot anytime she is forced to escape your tech-chase, if she's above you, or if she's offstage. Force her into these spots as much as possible.
Always remember you picked Falcon, you deal with Falcon problems. At any point if you complain about Falcon, you can always pick up Fox instead.