next time before you take something living, make sure you can take care of it. you could let them go, but there is always a risk that they will not adjust, i don't know, are they healthy, eating well? if so then you could let them go, make sure that your friend actually did find them outside, cause you don't want to introduce a non-native species, and check online or something if that kind of frog si native to where you live. even if you do let them go they could get eaten by another frog or a bird or something.