What will be the repercussions for an NCO who failed tape?

It's all in AR 600-9, you'll need to be familiar with that reg by the time you become an NCO. The regulation for flags is AR 600-8-2, which you may want to review to see how flagging actions affect a Soldier.
The Soldier will be flagged and placed in the ABC Program. They will have to continuously lose weight every month until they pass tape. They should receive a medical review to see if it's a medical issue, and be provided the services of a dietician. If the Soldier fails to make progress they will be separated. There are more specifics, but that's the overall process. When a Soldier is flagged they are not allowed to have any favorable actions such as using TA, receiving awards, or promotions until it lifts. When the flag lifts there is no lasting effect. However, if the Soldier fails again within a certain time frame, that is grounds for separation.

Five years ago I was taped and failed tape. It was a struggle to lose weight. I was not referred for medical evaluation. When I retired I had a physical a complete physical and not a Physical Assessment. I was diagnosed with a medical condition that causes weight gain. When I retired I had no flags. The condition was treated with medication. Had I been referred to a Doctor and received the treatment before I retired. I wonder if the attitude of the command would have been different?

He won't necessarily be behind if he's with the standards IAW AR 600-9 when he in the primary zone to be recommended for his next grade of
E-6; since he have just one (1) year time in grade it shouldn't effect him; however if he was promoted to E-5 late and has six (6) years time in service he may have a problem with getting a waiver to E-6 with two (2) years time in grade coming up on his seventh (7th) year for the primary zone for E-6. Double check the retention cutoff point (RCP), because it may have changed.