Washington is a no-fault divorce state. Basically, all that is necessary for a dissolution is for either party to allege the marriage to be irretrievably broken. "Factors such as adultery do not matter in determining property or custody issues" (research.lawyers.com).
A marriage, of course, must be based on trust. If the level of trust in the relationship has deteriorated to the point that one partner feels the need to have a private investigator look into the activities of the other, the relationship is in serious trouble regardless of whether or not the suspicions are justified. "Having proof" will not make any difference in the fact that the couple needs to either seek counseling or dissolve the relationship (in which case counselling is still highly advisable). As noted above, computer forensic services can be costly. In such cases, the money is better spent on counseling (either as a couple or individually) and/or legal representation. If these same concerns surface in an engagement, the couple is probably not ready for marriage. Again, personal counseling is strongly advised.
Accordlingly, this agency does not accept infidelity investigations. I do not want to take a client's money for something that will gain her or him no real benefit and may actually be damaging to her or his emotional well-being.