Rats should NEVER be kept alone. Just like humans who are kept in isolation as punishment, they experience depression, compulsions, and anger problems. Your friend should get her a healthy companion as soon as possible.
Just in the two weeks my spayed female has been alone since my old man died, she has began over-grooming and her tumor has nearly doubled in size (wish me luck at the vet this afternoon.)
Anything that is good for parrots is usually good for rats (both have high intelligence). I have what I believe is called a treasure chest (parrot toy) It’s filled with little wooden blocks that they have to get out through the different shaped holes in the sides. I like to stick treats in here as well, and it’s a hit along with newspaper, cardboard, and fleece scraps to make nests.
See if you can direct your friend towards gooseemoose. It is easily the best and most informative rat forum on the entire internet.