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First of all, the good news is that whether or not it is valid service, you don't have any obligation or liability if you're not named on the lawsuit itself. That is, if you're not a party to the lawsuit, any judgment against your legally separated wife will have no bearing on you, and you have no obligation to try to find her and give her notice of that lawsuit.
As far as this is legal and correct service, that's dependent upon the Utah Rules of Civil Procedure (it's Utah, not California, because that's the state where the lawsuit is filed). That's in Rule 4(d):
(d) Method of service. Unless waived in writing, service of the summons and complaint shall be by one of the following methods:
(d)(1) Personal service. The summons and complaint may be served in any state or judicial district of the United States by the sheriff or constable or by the deputy of either, by a United States Marshal or by the marshal's deputy, or by any other person 18 years of age or older at the time of service and not a party to the action or a party's attorney. If the person to be served refuses to accept a copy of the process, service shall be sufficient if the person serving the same shall state the name of the process and offer to deliver a copy thereof. Personal service shall be made as follows:
(d)(1)(A) Upon any individual other than one covered by subparagraphs (B), (C) or (D) below, by delivering a copy of the summons and the complaint to the individual personally, or by leaving a copy at the individual's dwelling house or usual place of abode with some person of suitable age and discretion there residing, or by delivering a copy of the summons and the complaint to an agent authorized by appointment or by law to receive service of process;
(d)(1)(B) Upon an infant (being a person under 14 years) by delivering a copy of the summons and the complaint to the infant and also to the infant's father, mother or guardian or, if none can be found within the state, then to any person having the care and control of the infant, or with whom the infant resides, or in whose service the infant is employed;
(d)(1)(C) Upon an individual judicially declared to be of unsound mind or incapable of conducting the person's own affairs, by delivering a copy of the summons and the complaint to the person and to the person's legal representative if one has been appointed and in the absence of such representative, to the individual, if any, who has care, custody or control of the person;
(d)(1)(D) Upon an individual incarcerated or committed at a facility operated by the state or any of its political subdivisions, by delivering a copy of the summons and the complaint to the person who has the care, custody, or control of the individual to be served, or to that person's designee or to the guardian or conservator of the individual to be served if one has been appointed, who shall, in any case, promptly deliver the process to the individual served;
(d)(1)(E) Upon any corporation not herein otherwise provided for, upon a partnership or upon an unincorporated association which is subject to suit under a common name, by delivering a copy of the summons and the complaint to an officer, a managing or general agent, or other agent authorized by appointment or by law to receive service of process and, if the agent is one authorized by statute to receive service and the statute so requires, by also mailing a copy of the summons and the complaint to the defendant. If no such officer or agent can be found within the state, and the defendant has, or advertises or holds itself out as having, an office or place of business within the state or elsewhere, or does business within this state or elsewhere, then upon the person in charge of such office or place of business;
(d)(1)(F) Upon an incorporated city or town, by delivering a copy of the summons and the complaint to the recorder;
(d)(1)(G) Upon a county, by delivering a copy of the summons and the complaint to the county clerk of such county;
(d)(1)(H) Upon a school district or board of education, by delivering a copy of the summons and the complaint to the superintendent or business administrator of the board;
(d)(1)(I) Upon an irrigation or drainage district, by delivering a copy of the summons and the complaint to the president or secretary of its board;
(d)(1)(J) Upon the state of Utah, in such cases as by law are authorized to be brought against the state, by delivering a copy of the summons and the complaint to the attorney general and any other person or agency required by statute to be served; and
(d)(1)(K) Upon a department or agency of the state of Utah, or upon any public board, commission or body, subject to suit, by delivering a copy of the summons and the complaint to any member of its governing board, or to its executive employee or secretary.
As you can see, there's nothing that gives them the right to validly serve process upon a spouse. It can be upon someone at the person's residence or usual place of abode, and IF she were living at your place of residence, then your acceptance of it would constitute legal service because you're of suitable age and maturity. But if she doesn't live there, the mere fact that you're still legally married has no basis on your ability to receive service of process.
Again, this imposes no obligation on you whatsoever. She can complain about lack of service of process and say that their service of process was not sufficient. She'd be the one to bring it up.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Look for the stars on your screen (★★★★★). Thank you, ***** ***** luck to you!