Husband has disclosed in writing that he has been soliciting prostitutes in the U.S. and BC

Probably a violation of the Mann Act. It's definitely not something you want to get caught up in. You should probably get a new husband; this one is defective.

Prostitution and solicitation are gross misdemeanors in Washington. Max year in jail + $1000 fine. First time offenders are typically made to get an HIV test and then can get a deferral where if you stay out of trouble for a certain amount of time it is dismissed. Washington is a no-fault divorce State so such transgressions are not supposed to affect the property distribution should you choose to file.

I agree with both of the above answers, however, without any corpus delicit, or corroborating evidence that a crime was actually committed, there may be no legal possibility for him to be prosecuted criminally. Just because a person admits to engaging in criminal behavior, that "confession" can only be used against him if there is independent evidence of the crime to which he is admitting. If there is no crime to which the confession can be linked, no corpus delicti, no prosecution.