This sounds like a well known and slightly recent glitch regarding falling through building pieces (which can extend to walking through/into walls):
The devs are aware and seem to be looking into a solution at very least. For the record, the issue is not isolated to just foundations as the post says here. It can cause phasing into walls and other building pieces too
Now why would this problem pop up for just for your friend? It could be false diagnosis and really be anything else similar to what others have said in this post.
Also, why is it just now popping up? The cause of the issue I mentioned is unknown, so I’m unsure. If it is in fact that problem, join the club of hoping for a clean fix (and soon!)
I feel it’s important to also say that there’s definitely more than one issue at play here, possibly including your friend’s connection to the server itself. The game in its current state also has more fun-dampening bugs than it has in the past, and while it will be tough to hold your friend’s interest, I wish you both the best!