if he's good at edgeguarding you there are 2 things you could alleviate the problem with:
1) get better at mixing up your recovery. sweetspot the ledge with illusion when there's no possibility she's in edgehogging position. ride the walls on FD and Dreamland with firefox to either sweetspot or pop up on the stage. sheik, unfortunately, is not one of those characters you can just fly above with firefox. she can tear you out of the sky with her fair in the blink of an eye, so take care.
2) get better at not getting knocked off the stage. this one is a little tougher, but learn to put yourself in winning situations. stay near the center of the stage when you approach. many of sheiks moves can be pretty easily DI down -> floorteched rather easily, such as a low fair or her dsmash. develop this ability, I promise you will not regret it. Finally, part of being good on defense staying on offense. fox can combo sheik to a pretty sick extent, but gets edgeguarded pretty easily, so stay in control.