There is a sticky at the top about this, but in general:
There should be a CM at the entrance who will tell you what you need for that ride. For most rides, the queue is accessible, everyone uses the same line. In some cases, near the end, you will be diverted for an elevator to avoid stairs, or to a different boarding area, either to allow extra time for boarding or to allow you to leave the
ECV at the exit point. In other cases, you can ride your ECV up to the loading point, and it will be moved to the exit point if you exit elsewhere.
In older rides, mainly in MK, you may be directed to go in through the exit or a separate handicapped line. If the stand-by line is long, you may be given a return time, so that you wait the same amount of time, but you can wait elsewhere. This is not the same as the
DAS, this is only for WC and ECV users, who don't need a DAS unless they have other needs.
A few rides, most notably POC, will require ECV riders to move to a wheel-chair. There will be WCs available. This is because either the ECV is too large, or there would be problems with getting the ECV to the exit point. For POC, the exit in an entirely different building, so they ask you to use their interchangable WC. (I don't know what they do if you have your own personal WC.)
A very few rides are not accessible unless you are ambulatory. Swiss Family Treehouse, or Tom Sawyers Island spring to mind, there may be a couple more.
If your friend can't walk a few steps to get transfer to the ride, there will be more challenges. Some rides vehicles will accommodate manual or motorized WC, but few if any can handle
ECVs. Some people can slide transfer into ride vehicles without walking, but CMs can't help you do that. They can offer you an arm to grab when stepping in or out, but that's it.
Again, the stickies go into more detail by ride, and there is a disabled user guide somewhere on line but I can't lay hands on it at the moment. Last trip I also noticed Disabled Guest maps in each park, that have accessibility marked for each ride, plus additional tips