We had a friend commit suicide the morning after our wedding, so things were... really messed up for a while in there. We'd also just moved into a new house, so we didn't have furniture arranged, everything was in boxes, etc. My mom was cat-sitting, and while she was gone, she did her best to make sure that when we returned home, it would not be to a house that was a disaster, but to a house we could relax in. We returned from the honeymoon to find that my mom had rearranged boxes a bit so that we could sit on the couch, had bought me some cute new kitchen towels for my birthday, had bought us flowers and breakfast and put them in the fridge, had gotten photos printed from our wedding and put some of them in frames that were wedding presents, cleared off the dressers in our bedroom and set them up, bought a coverlet for our bed and made it look nice, etc. Seriously, she did so much that when we came home, we were running from room to room exclaiming over new things we'd discovered. So I have no problem believing that the mother-in-law, if a hard worker, was capable of doing what was listed... especially if she'd planned it ahead of time.
The big difference between my scenario and this one that I see is that my mother did it without any snooping (she didn't open any boxes, for instance, only rearranged them to make furniture accessible) and without doing anything permanent (even using some wedding frames to put pictures in, we could easily just put different photos in the frames or even put the original packaging back together if we hadn't wanted to keep the frame), etc. Most things she did were *adding* something, rather than changing something or taking it away. Also, she is my mother, not my MIL. And no matter how much you love your MIL (and I do, dearly), there is a difference in relationship, especially if you aren't married or are newly-married, because you may not have known her long. My mother knew me well enough to know that I would be happy with her doing the things she did... my MIL probably wouldn't have known that, at the time.
If I were the OP in that letter, I probably wouldn't have had any problem with the MIL if she'd, say, gotten photos from the *public* part of my Facebook page (or what she could access if she were Friended) and printed them and put them in frames, then make a cute little display of them without actually mounting them on walls. The snooping aspects would have disturbed me (doing laundry without permission, going through clothes drawers, looking at my Facebook page from my computer where she'd get full access, etc.). And changing things out sends a *definite* message of "my taste is better than yours." It's very different from, say, buying a new set of linens, cushions, or whatever, wrapping them up with a bow, and presenting them with a receipt. That says, "I thought you might like these!" The other says, "I like these in your house so I'm going to put them there so it looks like I want it to." That sort of thing should *only* be done with permission and when the relationship supports it. My MIL has a much better eye for decoration than I do, so I often ask her for ideas and suggestions, and she loves helping me organize things. But I wouldn't just say, "Hey, can you do my kitchen for me?" because even though she does have great ideas, sometimes they aren't things I'd want to do. She often values the aesthetic over the practical, and I'm a practical girl all the way. Etc. So actually taking something away and replacing it with something else should never be done without being *asked* to do it, and even then should be done with care to make it reversible if possible until final approval has been given. This MIL was *way* out of line.
I like the idea somebody else had of having the boyfriend/DH ask other girls he knows. Girls, especially, because we tend to consider the house our domain more than men do, so we're more likely to a) want to make choices ourselves, and b) feel like our taste/abilities are being called into question by somebody making changes without our approval. Maybe writing down a list of all the things she did, so the boyfriend can't gloss them over and then claim that people agreed with him. That might help him to see that he might have been raised that way, but it's not normal. Definitely need to change the keys, but I do think it's a real eye-opener for a relationship, and she needs to make sure that she sees how he deals with this before she considers him husband material.