I refuse to stand in line for longer than 20 minutes, so to me going to WDW without a plan and during the summer is the end of the world. I would never to either. People that go without a plan and stand in 60 minute lines are always the ones who ask me why I love Disney so much because they think it's horrible. But, yes, not the end of the world. Thanks for putting it so nicely.
I understand your concern...but you mentioned you haven't been to WDW since FP+ was implemented. It's an entirely different thing than the old system. Plus, summer isn't really that busy, relatively speaking in the world of WDW. We are NOT planners, and we go every summer. Like you, we never stand in line more than 20 minutes, and we get on all attractions, including headliners. FP+ is a thing of beauty, IMO. If they're a week out, they may have a tough time finding things on
MDE now. But grab whatever is available. The morning of they should start checking MDE frequently. They'll find all kinds of things. We get FP+ for RnR, TT, EE, the Mountains, etc...just 5, 10, 15 minutes ahead of time, routinely.
So no, not the end of the world. For us, wouldn't even be a blip on the radar, it's how we normally do things. I think the best advice I could offer your friend is to fully learn and understand how to use FP+. If they do, they'll be fine.