Oh my goodness, I’m so sorry for your niece’s loss 
By this, I’m assuming that her best friend was also her pet. Losing a pet can be difficult, especially when we’re so close with them that they become like family. I lost a pet last year, one that I happened upon and rescued during Hurricane Elsa. He ended up with a blood clot [CW: pet death] that traveled into his brain and there was literally nothing I could do about it.
I think a remembrance jar is a great idea! One thing that I’m going to be making soon is a remembrance stone. We got a kit from the craft store and I can customize it for his name, the date he passed, and how much we loved him.
I found a few things here on the forum and on Spells8 to help ease the grief of losing a loved one, no matter if they’re fur-family or not.
@LadyDennaRahl wrote a wonderful to aid a dying pet and grieving owner that you might find helpful. It may need to be modified since they’ve already passed, but I’m sure it would be very helpful for your niece.
@SilverBear wrote a beautiful farewell ritual for a friend that passed away. It’s very simple and can be easily adapted!
There’s also the Wiccan Prayer for the Departed on Spells8. This prayer can be added to any ritual, including a memory jar, to make the ritual a bit more special.
Again, please pass my condolences on to your niece 
I’m so sorry for her loss