Looks like they have the first 3 books done..here is a set for them but you can find them per book also.
I've bought on Amazon over the last couple years they've been releasing. They are really cool, and I am looking forward to sharing the illustrated version with my children one day, luckily for my wallet, kids are hopefully pretty far off...
Here is a link for the first 3 books. You may be able to find used for cheaper and buy one book at a time.
I don't know if your nephew already has them but my daughter is around the same age and while we read all of the normal copies of the books first (we just finished Book 4 as a family) she really enjoys collecting the illustrated books
https://www.amazon.com/Harry-Potter-Illustrated-Collection-Books/dp/133831291X
They really did a nice job.
The illustrated editions are still being slowly released. They are really lovely. Here's a box set of the first 3.
Goblet is also out but not in the set for some reason (looks like it's out of stock on Amazon right now, though) and Order is scheduled to be released some time this year.
There is an illustrated Harry Potter book set that you may not know about. The illustrations are really nice. I've read the series more times than I can count and reading the illustrated books made the story come to life in a new way. I'll give you the link for books 1 to 3 on Amazon. I'm pretty sure they have the 4th one now but i don't have that yet if they do.
https://www.amazon.com/dp/133831291X/ref=cm_sw_r_cp_apa_glt_fabc_G2FDQ4X0XWXSXQDQ3R99
They are on sale for half off at the moment on Amazon
https://www.amazon.com/dp/133831291X/ref=cm_sw_r_cp_awdb_t1_7DypEb4G6WYJM
You probably won't be able to get her to replace them without really shaming her, but this way you might still be able to get them
First, your friend is being a jerk and you have every right to expect people in your home to respect your things, but Woah! $200 for just the first three?? If they're the regular Jim Kay version they should never have been that expensive, although they're beautiful. Here's a link to a box set of the first three on Amazon for $65. Send it to your friend, because she absolutely should replace your books. I'm sorry that happened to you :( Harry Potter: The Illustrated Collection (Books 1-3)
Probably too late for you at this point, but I recommend the illustrated editions that they have been coming out with. They've been coming out with one new book a year, and unfortunately they only have the first 3 books, and the 4th one is being delayed (because it's so big), but we really enjoyed them and I think they are a great for young kids. Here's a link:
https://smile.amazon.com/Harry-Potter-Illustrated-Collection-Books/dp/133831291X/
Harry Potter: The Illustrated Collection (Books 1-3 Boxed Set) https://www.amazon.com/dp/133831291X/ref=cm_sw_r_cp_api_glc_fabc_OPFaGbR1RZ0RQ
Amazon. But only the first three books have been illustrated so far.
There’s two versions, one by Jim Kay and the other by MinaLima.
The Jim Kay ones are up to GoF - https://www.amazon.com/dp/133831291X/
MinaLima published the first book last year, CoS coming out some time this year - https://www.amazon.com/dp/1338596705/
Both look amazing but the art styles are completely different. The Jim Kay books are larger in size (like a coffee table book) while the MinaLima book is a regular book size. Jim Kay’s art is a bit darker while MinaLima is brighter. And MinaLima has interactive illustrations which are pretty cool.
Harry Potter gets pretty deep for a 6 year old around book 5, but your call...
You could slow it down by reading them the GORGEOUS illustrated editions. They had been doing them once a year but have slowed down now. Only the first 4 have been completed. I'm hoping we'll get book 5 this year.
https://www.amazon.com/dp/133831291X/ref=cm_sw_r_cp_apa_glt_fabc_GSFPWHFSD02C5V6TPE8Q
https://www.amazon.com/dp/0545791421/ref=cm_sw_r_cp_apa_glt_fabc_T749WBNBYB9E78Q7VZE4
And Amazon has the first three in a bundle for cheap: Harry Potter: The Illustrated Collection (Books 1-3 Boxed Set) https://www.amazon.com/dp/133831291X/ref=cm_sw_r_cp_api_i_VUvBCbNJ9RJ86
Harry Potter: The Illustrated Collection (Books 1-3 Boxed Set) https://www.amazon.com/dp/133831291X/ref=cm_sw_r_cp_apa_i_0SCWDbFMSV8BW
They're gorgeous. Warning: there are spiders ALL OVER the 2nd one.
Harry Potter: The Illustrated Collection (Books 1-3 Boxed Set) https://www.amazon.com/dp/133831291X/ref=cm_sw_r_cp_apa_i_P123Cb505XW46