I am not the expert you are looking for to answer this but I did see in a recent shooting sports magazine that the NRA style matches will this year have a tactical rifle match. I would assume therefore there are 2 type of matches he may be referring to that the AR type rifle would be used in.
As the thread title states Service Rifle I would recommend one of Johns' uppers. White Oak Precision - Service Rifle Uppers
Lower doesn't really matter as long as it has a good trigger in it and it complies with the Service Rifle rules.
A Bushmaster or a Rock River Service rifle will do just fine. A White Oak will do more than just fine if the shooter is up to it. I love mine.
How serious about it is he? If he is just going to shoot reduced courses, he can do just fine with a standard 20 inch 1X9 barrel. A free float tube, RRA trigger, and 1/2 minute rear sight will get him going. 69 grain handloads will perform out to 300 yards.
I also suggest that he attend a Highpower Clinic. There are always loaners for new shooters. Wildcat has one in the Spring and FWRR will have one May 7 this year. He can decide on a service rifle after shooting the rifles at the clinic.