A bit of background
I'm a junior female developer working for a telecommunications company. Shortly after I started, another young woman joined the team. Our manager sat us together with the intention of doing some pair programming. As we are the only two women in the team and we have similar interests, we soon became very close. Our tasks are mostly bug fixes and small non-critical new features on a legacy system that will be replaced in a not so distant future.
However, as the system we work on lacks any kind of documentation and neither the senior developers nor our manager are always available to help, we sometimes make mistakes. Most of the time one of us spots them, but there have been a few times where not even testing noticed, and everything blew up in production. Last time it happened our manager decided to split the team, and placed us in different areas of the building. We cannot see each other as frequently as we used to.
Last week our manager approached me. He was curious about some changes that were applied without his permission. When I made clear that none of my tasks involved the files in question, he had a meeting with my coworker in a nearby office. It was a heated and loud discussion, in which he accused her of various nasty things I didn't know and didn't want to know. By the next day the whole department knew about their argument, which got worse after she reported him to management, who in turn called him out for his outburst. After that, he told me and the other developers nearby that we "have to let her make her own mistakes", and explicitly told me to not help her in any possible way.
Now, it's clear to me our manager is evaluating her performance with caution, even gathering evidence in case he decides to put her in a PIP (Performance Improvement Plan). While I know this is not my business, my coworker still asks me for help, and I'm running out of excuses. At first I told her I was busy, or suggested to look at my notes (which are located in a shared drive) or to have a look at the code itself, but I'm afraid that she doesn't take the hint.
How do I stop her or convince her to stop asking for help after I've been told to let her work on her own, without sounding rude, malicious, or that I want her to be fired?
Edit: I talked with our manager at that time, and he brushed off my concerns saying "just tell her you're busy". Just in case, we've updated our résumés and started browsing job sites.
I know there are questions like What can I do to make a coworkers lack of effort more visible?, but IMO they focus in whether the OPs should report their coworkers to their Managers, and not how to deal with this kind of behaviour.
Update: A few days ago I came back from my vacation and to my surprise I found out there was a reorg while I was away. The whole team was relocated to another floor, the PM resigned, my manager was returned to his old developer role (apparently by his own request) and my friend and the head of Testing put in their notice on the same day. I don't know if all these changes (especially my manager's demotion) are consequence of the incident, but it seems to be resolved.