It's really down to the risk, I'm sure you're friend doesn't want a DSC completion more than they would like to keep their account.

this is what bungie has to say. i would believe you fall in the 1st paragraph... but that's my opinion [quote]Account Recoveries Account recoveries are when a player gives someone else access to their platform account and control of their Destiny characters. This not only puts your account security at risk, but it can also have adverse effects on other players’ experiences in competitive matchmade activities. Starting this Season, participating as a buyer or seller of account recovery services could result in a ban on your account. While we will only be placing bans or restrictions for certain circumstances, we strongly recommend that you never give your account login information to anyone else. Not recommended, but won’t get you banned: Sometimes I let my kid grind Lost Sectors. I asked my cousin’s friend to recover my account to check Xur because I was stuck at a wedding in a remote area with no internet access. My roommates and I all share an account on the common room’s console. I let my friend borrow my account to try out a weapon roll. What could get you banned or restricted: Charging others to recover their account for boosting, earning items, completing content, etc. Recovering accounts in exchange for anything of value including but not limited to Patreon supporters, subscribers, follows, gift cards, etc. Paying someone to recover your account to be boosted, have items earned, etc. Boosting recoveries done for free can also result in bans or restrictions. What could happen if I pay someone to recover my account? Temporary or permanent bans may be applied to your account if you are using paid recovery services. You are responsible for the security of your account and bans will be applied if someone else is detected cheating on your account or your account benefits from being in a fireteam with other cheaters. Bungie will not recover any lost items or progression if you or the person/service you share your login information with deletes anything. This is not a comprehensive list of all circumstances that could result in bans or restrictions and Bungie reserves the right to action accounts at our discretion. [/quote]

No. That’s not bannable.

Sorry but Bungie had to come down hard in recoveries. Too many are charging money for recoveries and they all cheat causing more problems. Every day this forum page was full of recoveries gone wrong all expecting Bungie to fix the issue. If people can not follow common sense and advise from Bungie, Sony, Steam, Microsoft and even governments then sometimes harsher means to protect people's accounts are needed.