What kind of photography gear is she using now? Does she have plans to upgrade that in the future? Does she shoot RAW or .JPG? These factors must be considered, too, prior to purchasing a new computer. If, for example, she is currently using or planning to purchase any full-frame camera which has 50- 60 MP and she currently or plans to shoot in RAW with that camera, she needs to keep that in mind while making her purchase of a new computer.
In addition, since you mention that she shoots wildlife, does she shoot many frames per second (20-30 fps) in Continuous High+ mode? That can make a big difference, too, when it comes to transferring all those files into a computer. That computer needs to be able to safely support and manage those files.
The new computer should have plenty of "oomph," a powerful processor with the ability to easily handle many large files, and it should have plenty of space to accept and temporarily store large photographic files, and it also should have plenty of RAM to easily handle the reviewing/culling/editing process of those images.
A few years ago when I was considering moving from using DSLRs to using mirrorless cameras, as I was reading reviews and information about the options available at the time I quickly realized that first I was going to need to update my computer in order to have one which would handle what I was anticipating would be an increased workload on many fronts. Did that, then made the purchase of the new camera gear. Two or three years after that, now having had more experience in working with the larger files and also paying attention to my somewhat changed shooting patterns and such, I again updated my computer, this time significantly increasing both the storage space and the RAM in particular. I had found that I needed to do so.
And, yes, in addition to the computer itself, external HDDs and external SSDs are one's friend, both for backups and for everyday use in various ways in order to take some of the load off the computer. I prefer to review, cull and process my images in the computer but then shift the edited files to an external drive. Actually before I even start doing anything with those files, though, I immediately copy any card filled with newly-shot images right to an external SSD to ensure I have a backup. Everything is backed up and eventually archived throughout the process. This requires plenty of storage both within the computer itself and with the use of external drives.
Of course your sister knows her own needs and preferences better than any of us here but since you asked on this forum for advice but without providing us much in the way of information about how she deals with photography now, what gear or accessories she is currently using or planning to use and how that will have an impact on whatever new computer she finally chooses.
Hope this helps provide some things for her to think about....