You can suggest a better approach. In most cases, the person would understand the mistake and see your suggestion as an act of improving the junior dev's knowledge. If they don't understand, you can show them pros and cons of both approaches. A bad approach will definitely have a lot of cons.
As a senior dev, one should understand that a less experienced dev may not have seen multiple approaches yet. They might have just seen one approach and tried to implement it. So being blunt with a junior dev, especially a new joiner could demotivate the person. Also remember the junior dev most likely does not "own" the approach. So avoid using words like "your approach". Focus on the bad things in the approach and not the person.
You can be blunt with colleagues who are also your close friend, when you know how they react to criticism, but being blunt with someone you do not know much can have a lot of negative impact on the person's mental health.
For further actions, you may consider blocking this person and/or reporting abuse
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You can suggest a better approach. In most cases, the person would understand the mistake and see your suggestion as an act of improving the junior dev's knowledge. If they don't understand, you can show them pros and cons of both approaches. A bad approach will definitely have a lot of cons.
As a senior dev, one should understand that a less experienced dev may not have seen multiple approaches yet. They might have just seen one approach and tried to implement it. So being blunt with a junior dev, especially a new joiner could demotivate the person. Also remember the junior dev most likely does not "own" the approach. So avoid using words like "your approach". Focus on the bad things in the approach and not the person.
You can be blunt with colleagues who are also your close friend, when you know how they react to criticism, but being blunt with someone you do not know much can have a lot of negative impact on the person's mental health.