Well, your friend can't get a declaration that his wife is presumed dead and killing her is not a practical option so, if she's refusing to acknowledge service, he'll have to go down the route of proof of service by an official ('bailiff' in Sycamore's post). The courts are used to the other side playing games in divorce cases and have weapons and experience to thwart them. She can't then deny that she's had the papers.
In extreme cases, where the other party has conveniently or mysteriously disappeared, it is sometimes possible to have what's called substituted service on the other side in civil proceedings. That is usually effected by advertising the proceedings in a newspaper relevant to the area where the person is believed to reside and/or nationally so that they may be deemed to have notice. I once saw such a notice in a magazine for dog lovers; that shows ingenuity, but the other party must have been a known subscriber. Never seen it in divorce cases; dare say, if possible, it's never needed (see above "playing games").