my best advice is dont do it ... or better.. dont release it into your garden ... grow it in a pot ... or you will spend the rest of your life trying to get rid of it ... been there.. done that ...

that said... mint is neat ... i used to grab a leaf or two and put it between my cheek and gum .... nice fresh taste.. and no sugar ...

seriously .. grow it in a pot after it roots ...

if i were your friend.. i would have gently tugged at my plant.. and a rooted piece would have come right off the plant ... it has runners that are growing undergroud all over the place ..

ken

I agree with Ken about keeping it in a pot but...it roots very easily so I would
1 take a small pot (2-4")
2 fill with potting soil or vermiculite if you have it
3 moisten soil or vermiculite (not drenched)
4 using a pencil make an indentation in the middle at least half way into the soil
5 with the plant piece in hand, cut right below the bottom node (where the leaves attach to the stem)
6 if you have rooting hormome dip the bottom two nodes into the hormone, shake off excess
7 put plant piece into indentation in soil
8 lightly press soil around plant
9 water lightly
10 take a gallon size ziplock bag and put pot and plant into bag.
11 close bag all but 2" or so (for circulation) but to keep pot from drying out to fast.
12 place bag & pot in indirect light until new leaves appear (this could take awhile)
13 make sure soil doesn't dry out and that too much moisture doesn't collect on the bag. If too much moisture collects open bag completely until moisture is gone, then reclose partially. (You may get conflicting arguments on whether the bag should be closed or not, that is why I leave it partially open...mostly experience-I've lost a couple of scented geraniums with the bags closed and then forgot them)

But then that's just my opinion.
Karen

Ditto on the pot suggestion. Mint is a wonderful plant to have, but it is extremely invasive because it shoots runners under the ground. If you can go back to your friends house, ask them to yank out a few pieces with the roots on the ends so you can pot them up sooner. I would leave the cutting in the water, just for the fun of the experience.
Annie