Does she meet the wrong guys or is she attracted to the wrong guys? Perhaps she needs first to think about what was wrong with the previous men she had relationships with and why she was attracted to them and then adjust her mind set so that she is not attracted to the same type again.
Perhaps, also, if she was less worried about being single and just made a life for herself where finding a partner wasn't central to her life, a partner would be more likely to appear, there is nothing more offputting to a potential partner than someone desperate for a relationship.
My DD was 40 in 2013. She decided some years ago that life for her as a single woman was actually more attractive than finding a partner. She owns her own home, has a job she enjoys, as many friends as she wants and a host of hobbies and interests. Her niece and nephew adore her and ignore parents and grandparents if she is present. My sister was similar - then married at 47. Whether DD marries or not she has made her choices and must live with them
Anyway, any perusal of Gransnet threads shows that marriage is no guarantee of happiness for Gransnetters or their sons and daughters. Perhaps you should stop worrying about your daughters single status and encourage her to enjoy the life she has, instead wishing it away for something that may never happen.