I have a Nikon 3100 and a couple lenses.  I offered it to my daughter, but the Nikon is probably 6-8 years old, and now I'm thinking I should just buy a new dslr for her.  I am looking at the canon 80D.  Is the 80D a step up from the 3100?  Advantages or negatives?  Thanks.

What is your daughter to going to use the camera for, personal trips, real estate photos, picture of her children etc?

Travel and casual wildlife photos.  Birds, etc.  

Now I'm wondering about mirrorless.  The CANON m6 mark II?

Travel... mirrorless.
Birding & wildlife... 80d.
But that's just my opinion.
Notice I didn't speak to Nikon in this mix because I haven't used a Nikon since the days of film. So I don't know their digital lineup at all.

I've been mostly Nikon SLR since the early 80s, and canon for point and shoot, but Nikon politics regarding guns is objectionable now.  Maybe canon is similar but I haven't observed it directly.

Personally I don't believe in wasting money, but in using it wisely. That said, I would lend your daughter your Nikon 3100, and have her take bunches of photos with, like to go the park, around town etc, do a bunch of snaps. And see how they come out.

I still have my Nikon D40X which is like 2 generations older than your 3100,  I personally put on 150,000 clicks on the shutter, and it still functions today,  well more or less, it needs new batteries because it is so old, that B&H, Aldorama etc only carry Nikon batteries at ~$50 each.