adding friend into gym

This is "Account sharing", against the TOS. A lot of people do it anyway and catch that rare mon for their girlfriend and their nice too, haven't heard about one being banned. Putting them in gyms is a different thing, at least put them only in slots you created by prestiging it up.

Both of those actions go against the terms of service and you shouldn't do it, but it's really up to you, your account it's not getting banned, your friend's account migth, but not for account sharing as that's kind of hard to prove with 100% accouracy, however, depending on how you do it, he/she might look like he/she "teleported" and he might get a ban/soft ban for spoofing.

I assume this applies to using 2 accounts on one phone. If your friend lends you their phone for the purposes of walking eggs, catching mons, etc. I don't see how this could possibly be a problem but someone please correct me if I am wrong...

I do this for my son (using two phones) on a daily basis to help keep his Pokestop streak going and to get him into some gyms. I will take down or prestige a gym to open up a spot for him.

The only way it would be a noticeable problem is if you're both logged in at the same time at two different locations. That happened to me once and I just wasn't able to interact with any stops or 'mon. No ban.

If you do this, make sure that your friend is not currently playing and they have been off long enough that they could have traveled to your location during that time. If you are doing this to catch a rare pokemon, you should try to spin a pokestop or catch another pokemon first to ensure that the other player has not been softbanned for "teleporting" to your location.