Whew.....
The solicitor gave advice whereby she could technically stay within the law. True enough, a Europe trip will satisfy the requirement to leave the UK before your visitor's stamp expires. You ARE NOT required to go back to your home country, only out of the UK. The solicitor was correct. However, it's also true that technically, you can only remain in the UK on a visitor's stamp for 6 months a year, regardless. They sometimes overlook this and stamp you anyway (they did this for a good friend of mine who was visiting me, went to France on business, and came back a few days later--she got a new 6-month stamp and she did not ask for one nor did she need one). However, you can by NO means count on them overlooking the 6-month total requirement. Your partner may well have faced the same problem if she had gone back to the States and come back 2 days later, instead of going to Holland.
I am very surprised that her solicitor did not warn her of the risk she was running if she tried the Europe hop a second time. When I asked about building up time towards the unmarried partner visa, my solicitor specifically warned me against doing what your gf did. He said that some people get away with it once, but after that I was likely to be heavily questioned at best, immediately deported at worst, and he could not in good conscience recommend this strategy to anyone. UK Immigration are under a great deal of pressure to turn away anyone who strikes them as having even the slightest bit of improper intent. Unfortunately, this means they are often giving a hard time to people who simply want to live here in peace with their partners but have not yet built up enough living-together time to get a proper UP visa. It's a catch-22 and I feel for you...I went through this myself...they say you must prove 2 years of cohabitation but they make it impossible for you to be together in the UK long enough to do that, unless you can get a work permit or have access to enough money to get a student visa.
What's more, they do not apply the rules in any kind of even-handed manner. My friend got a second 6-month stamp (without even wanting one, mind) coming off the Eurostar as a routine matter--my theory is that because she was white, fortyish, and professionally dressed, she did not fit any of their profiles and so they didn't even look twice at her. They deliberately target certain people for questioning. In addition to the much-ballyhooed racial profiling, which occurs even though it is officially denied, other frequent targets are: people who admit to being in a gay relationship with a Brit (to see if they are trying to build up time towards the UP visa while on visitors' stamps--techincally, that's "intent to settle" and is not allowed, although some gay couples have had success on appeal because of the catch-22 nature of the requirements); young women traveling alone especially outside the "student" seasons (to see if they intend to marry in the UK without getting the proper visa first); young men and women who don't appear to have a lot of money but who state that they intend to visit for a full six months (to see if they intend to get a job in the UK without a proper permit); and anyone who seems, from their recent immigration history, which they see on the computer screen when they run your passport through the reader, as if they may be trying to settle in the UK on a visitor's stamp.
Now that Immigration has given your gf an official hard time, she may require some sort of entry clearance to get back into the UK, even as a visitor. If she is going to be divorced soon, my guess is that your fastest option to get her back into the UK would be the fiancee visa. If you two don't want to do that, then either you will have to endure a long distance relationship for a while, or she will have to look into student status, work permits, long-term volunteering (tough to get, but doable), or one of the special immigration programs such as "highly skilled" if she qualifies for them.
I'm not trying to be harsh but this is the reality of dealing with UK Immigration.