As bouteloua and conboy said, using a cable/charging cord to connect the phone to a PC isn’t difficult and is definitely the most efficient way to transfer large numbers of photos. Even 200 or so should only take a minute or two. Personally I don’t do any kind of cloud storage of my photos, so I transfer them over cables all the time, to back them up on my computers and an external hard drive. If I transfer excessively large numbers of photos, I’m talking like 10 years’ worth of photos, it does start to take a while, maybe 20 min, but I assume your friends aren’t doing quite that many!
Since your friend’s photos are already geotagged (the location is saved in the photo file), the uploading to iNat step could end up being the limiting factor. If the internet connection is slow, it could take a long time. If the computer isn’t very new/good, it could freeze or crash the browser. Or they could make a mistake and delete all the photos they just uploaded without posting them first. So for those reasons I recommend doing batches of 20-30 photos at a time, or however large a number seems to go smoothly without too much risk of failure–definitely less than 50 at a time! Adding IDs could take a lot of time as well, depending on how into it they’re going to get.
It’s great the photos are already geotagged. Me, I’m typically using a camera that doesn’t geotag and then I have to spend effort getting them tagged before uploading.