Tell them both NOT to take help of anybody else including you or parents, relatives, friends, police, etc etc(don't feel offended but its a truth).
And request all others not to indulge into it. What traspires between husband and a wife always remains unspoken and it them both to resolve it. All other can only add 'pretol' in the fire.
This is the best thing that they should know themselves, if one doesn't want to pay heed to this imp thing, then both suffer....it's not a myth but truth.
IF they both decide to take help of a counsellor then only it works, any third person asking/trying for that.............. it causes either one or both to resist.
One can only suggest both simultaneously to seek help of a private councellor, and no interference of any third person there.