It sounds sensible to me that they came to a financial arrangement and she was able to move out. After living together for 10 years she would need to be able to rent and would need a deposit, moving costs, etc. The 10K may have just been a token to the man or a significant financial burden, but we cannot say, and we cannot say what promises they made to each other over the 10 years. The woman may have fretted that she never contributed, the man may have promised to “look after her” if they ever split up, etc, etc. it sounds like they had a clean break for a relatively minor cost (for 10 years of living together).
As for your friends daughter, the solicitor was being prudent in pointing out future pitfalls to her. If she moved in the love of her life, she could do anything from marrying him, to asking him to pay rent, to insisting he not spend a penny, and all have some financial implications if they broke up.
Living together is a very strong statement of life commitment to each other, and the dissolution of that couple can have all sorts of ramifications.