Someone I know was banned for improperly tagging their art. They asked me if I would upload their art for them and I'm just wondering if I would get in trouble for doing that even if I followed tagging guidelines?

No.

I'd think that simply uploading a banned user's art would be fine if you followed the tagging guidelines, but there is a rule against circumventing bans that may be applicable.

Circumventing a Suspension / Ban
Suggested Suspension Length: Permanent
This category includes:

• Posting anything on another account
• Having someone post on your behalf

If someone is found to have violated this rule, they may be banned with no appeal, regardless of previous disciplinary action
Alternate accounts are generally okay, so long as they are neither used to evade a suspension/ban, nor to abuse the vote tools

Fairly self-explanatory. If someone gets Suspended or Banned, and they try to go on a friend’s account, or alt account, and post anyway, that account gets banned as well. The other side to this is if they were under a short-term suspension and they go around to circumvent it, that short-term suspension becomes a permanent ban. If a user does this, but acts in such a way as to not continue the actions that got them banned, then an admin may choose not to enforce this rule, under the guise of helping the greater good.

(source)

Alright, thanks for the info! I'll probably not do it since it kinda made me uncomfortable anyway.

renwinterfox said:
Alright, thanks for the info! I'll probably not do it since it kinda made me uncomfortable anyway.

Simply posting art that you think is good isn't circumventing a ban.

furrin_gok said:
Simply posting art that you think is good isn't circumventing a ban.

But if they're being specifically asked to post on the banned user's behalf... The admins most likely won't do anything, but I get the hesitation.

I think there is more leeway to the rule in that it is more directed towards posting comments on their behalf.
I don't think posting their artworks would inherently be bad, unless their works are the cause of them getting banned in the first place; though I will not be going ahead to test the idea.

renwinterfox said:
Alright, thanks for the info! I'll probably not do it since it kinda made me uncomfortable anyway.

you should absolutely avoid posting the art if it makes you uncomfortable, big shoutouts to you for avoiding posting it due to potential guideline problems.

I think it will be better to directly ask an admin.