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.

The course instructor is the person ultimately responsible for all aspects of the class, including proper implementation of departmental and university policies. As a TA, it is not your role to decide which policies should be followed and which can be ignored. Therefore, regardless of your personal feelings, you should report any issues that arise to the instructor.

As others have said, it's important that the instructor know about this. It is ultimately her responsibility to handle the situation, and your job is to make sure she has the information she needs.

Note, though, that just because you report it to the instructor doesn't necessarily mean that a formal disciplinary case will result. The instructor may decide to handle it informally, by having a talk with the students about appropriate and inappropriate collaboration, etc. Under the circumstances, I think that is what most people would do. Note that such a talk will probably have more of an impact on the students coming from the instructor instead of from you.

When you report the matter to the instructor, you can certainly point out that it is the students' first college assignment ever, and that you believe leniency is appropriate. But of course it is the instructor's decision in the end, and you will need to respect it.