How long have you been friends instead of therapist/client?
Have you told thia ex-therapist/friend about your husbands worry and your questions about the relationship? Other than your concern she will leave?
Here are some articles that help explain what might be going on that would help explain why this is so intense so fast -
Desire for a personal friendship with my psychotherapist
Relationships after Therapy: Why They are Always Bad « Shrink Talk
Therapists can do a lot of sketchy things in therapy, and not lose their license. Dual roles like how this friendship started are illegal. Not just ill-advised, but flat out illegal. Part of why it's illegal is because sometimes therapists get caught up in their own stuff and it would feel like a good idea to be friends with a client while they are in therapy, and then to end the therapy so that they can be friends with a client. Even therapists need the reminder, the clear boundary, no don't go there.
But she went there, knowing it was against the law, knowing it could end her whole entire practice, and your therapy with her fell apart.
As would be expected.
You stayed friends, but it's not your best interest she always had in mind. Oh I'm sure you both feel like she has been good to you and been your helper and your friend - but there is likely a counter transference /transfeence dance occurring.
She isn't in a place of health. You two continuing this relationship at this depth and with this kind of enmeshed and almost obsessive intensity isn't good for her or you.
This isn't meant to put you down, but this isn't likely all about friendship for her. There are likely issues she is trying to work through as well, that she should be seeking out her own therapist to address. She listens to client issues all day, deals with intense closeness and emotions all day, and during her time off, she seeks out connection with a former client, requires honest openness about everything, tells the client she loves her and etc... ? Something is off for her and letting this go on and on just how it is isn't healthy for her or you.
Just the simple fact that at any point in time you could have her license taken from her by simply telling authorities how this relationship started puts weird elements into this relationship. The fact that she is requiring open honesty at all times without any legal backing of privacy like in therapy shows there are boundary issues running amok.
It is very hard to back out of relationships like this (which is part of why it's prohibited to be friend with your clients that you are treating in the first place.)
It can feel impossible and that is normal for this kind of relationship --- and it's why they are so problematic.
You are much more vulnerable of being taken advantage of in this relationship than others, and you are naturally worried and spend a lot of time in this relationship on very deep levels.
I would really suggest being as open and honest as you can with your therapist about how worried you and your husband are about the quick deep intensity of this relationship and stay connected to an outside professional. I continue to recommend taking a little space from this relationship. Don't get more addicted to the powerful feeling of false intimacy that comes with being so enmeshed with another. I've been there and it nearly cost me my life. Don't let her be your everything.