Weird. I'm going to echo the rest and say stay away from it. The only thing you should do is refer her to HR and do not provide any input at all unless specifically asked by HR/Legal.

If she applied for the job and it specified that she'd be using/supporting Microsoft products, they'll handle it. If she misrepresented herself in her interview, they'll handle it. If it is not seen as a reasonable accommodation, they'll handle it. Religious exemptions are pretty clearly defined by the DoL, but that's for HR/Legal to decide.

This is a law firm in your state that details what they'll be looking for: https://www.jfbeasleylaw.com/religious-discrimination.html

You may get called in to see if it is technically possible to support her request without "undue hardship" (they will define it for you) given your business tools and infrastructure, but do not offer that information ahead of time. Wait for them to ask and be very, very clear with your answers. Do not waffle on it. Legal will tell you that "Yeah, I mean, we could, but..." isn't acceptable in a situation like this. It needs to be a hard answer.

Be prepared to discuss support, management, security, any duties that may be affected by software restrictions should she be using a different OS than the rest of the office, whether or not it'll make it difficult for others to do their job because of her accommodation, whether or not there will be a downstream impact due to her accommodation and stuff like that.

Here's hoping for an update. I'm super curious how this will play out.