How should disallowed collaboration on a freshman homework set be handled by a TA?
I personally follow the rule "once is nothing, twice is a habit". I think a warning would be a good way to start. I do, however, think that it is still a good idea to run this by the instructor. I assume he/she is responsible for the course and the decision is up to him/her. You can explain your point of view and perhaps get some feedback on the matter. In addition, by taking up the discussion the problem is in th eopen and it makes it easier to catch repeat offenders. If you keep it secret, the culprits, in the worst case, can get by repeated cheating for each new teacher they encounter.

Cheating is an abomination but I think people are allowed to make mistakes. I also think most people learn from mistakes and will not easily do it again. But, it is necessary to make sure they understand their attempt has put them under scrutiny, that is probably "punishment" enough.