Well, tell her you simply don't have time at the moment, with your separation and house-hunting etc, and that she'll get a faster, more in-depth critique from someone else.
I think it's fine to ask a friend, assuming that friend is literate, reads a lot and, especially, is well-read in the genre you work in. If you write psychological horror, there's no point in asking your friend who only ever reads Marian Keyes for a critique.
And I think you need to be sure that your friend is (a) any good -- I wouldn't be taking time to read unedited freeform gabble and (b) that it's already been through multiple edits before you see it.
I exchange MS with a novelist friend, but we only send things to one another at the stage where we would also be sending them to our agents, so are at a high level of finish.
And (c) you need to trust the friend as a reader, and both of you need to be able to be brutally honest and maintain the friendship -- for instance, I had to tell my friend that her use of dialect really didn't work at all in one of her novels. I was right, and her agent agreed, but it involved a complete rewrite, so she needed to trust that I wasn't wrong about it.
And she told me that an entire subplot didn't work, and I removed it.