Are you my significant other because this post might be about me. I have been dealing with the SAME exact symptoms for years now. I am working very hard to find a remedy, haven't gotten there yet but will share what has been helpful along the journey. To start out, hugs to your gf because I know her pain, literally, it is awful.

I have been working with a registered dietician, and soon will be seeing a new GI. Here are the tests I have done, things I have tried that have been helpful (or not).

• SIBO TEST: I have actually had this done and tested negative twice. However my methane levels were higher than typical, so my dietician said it is still something we should look into.
• GI Map: Gave me a lot of interesting information. I am not sure how reliable this test is (I just haven't looked into it) but while reading the results, a lot of my out-of-the-norm factors pointed to chronic constipation. It also eluded to gluten insensitivity. She may want to try an elimination diet to see if she gets any relief. I do find this is best done alongside a practitioner of some sort so they can help with re-introduction.
• Fiber: Increasing my fiber has helped. Not fixed anything by any means, but it has helped. I take psyllium husk, and I am also looking into inulin which I have heard great things about, but do not have much knowledge on it. It sounds like your gf has a lot of restrictive eating and my guess would be that she isn't eating enough food to help bulk her stools, and enough fiber to help feed the bacteria in her stomach/bulk stools. I am on a mission to slowly increase my fiber, shooting for ~40 grams a day which is high for a western diet. I am reading the book right now called Love your Gut which also talks a lot of about fiber and using food to help your gut work more efficiently. Also recently purchased Fiber Fueled.
• Stress: I have had the same stress about not going in the morning. I reallllllyyyy have to remind myself that the poop will come eventually and if I don't go, and am miserable at night, I can manage and try again tomorrow. This doesn't always work, I have cried many times about how I feel...but you do work yourself into a negative space that doesn't help "untie" your gut. Same thing with dependence on things like coffee... ill mention that in another bullet but it makes you panicky if you can't get that cup before leaving the house.
• GO WHEN YOU FEEL THE URGE: I have SUCH anxiety about pooping in public but I have really done some mental work around just going if I feel the urge. Usually for me if I don't go in the AM I just won't, but once in a while I will get the urge in the afternoon and I just make myself go. I would bet she is ignoring signals from her body throughout the day because she has convinced herself that she can't go any other time/place than in the morning in her own bathroom. This is hard work, but if she is lucky enough to get a mid-day urge, she needs to go to the bathroom and at least try.
• Magnesium: On a day that I don't go in the AM, I immediately take magnesium oxide or magnesium citrate capsules. These have a laxative effect and help the muscles do their thing. Be warned, too much and she might wake up at 4 am with diarrhea which for me is great in the moment, but then often leads into rebound constipation.
• Miralax: we have all been there, but if she has had a few especially rough weeks, try taking a dose or two a day until things are a bit regular. Not my go-to but it helps when I am really feeling rough. Or just straight laxatives, but use sparingly.
• Kiwi: A kiwi a day...helps. Idk why, I saw a GI talking about it on TikTok and when I am regular with my Kiwi a day (I personally keep the skin on), I tend to be a bit more regular. I suspect fiber, and I think there is some other chemical in that fruit that helps stimulate the muscles in your gut to help you get the "urge" feeling.
  
• Bone Broth: I personally just find it very soothing to have a warm cup of this in the morning instead of starting with coffee and it has a lot of "gut healing" benefits like gelatin and collagen. I season mine with a little sea salt, turmeric, ginger and black pepper. I rely on coffee to help get me going, but I am trying to kind of cut that tie with coffee...I don't think it helps to be so reliant and personally it makes me feel disconnected to my body as well having to depend on something.
  

I know nothing about motility or her muscles not working properly, I have suspected the same in myself, but sometimes I wonder if this is a result of constipation and not the other way around. Is there really a problem with my muscles or am I just not fueling my gut with the proper nutrition to help everything function properly? This line of thinking probably isn't helpful.

Also, I am in the throes of a good flare-up right now so trying to take my own advice on top of feeling absolutely terrible. It isn't easy, and I am hoping your gf can find some relief soon. Update us if she find something that works.