Quoted from ReadyPO:Well, sorry to hear this and I agree with EJS , Silver (or Gray) Divorce is not all that uncommon these days. Living longer and healthier is one reason, combined with all kids gone out of the house, leaving a void that needs to be filled with other common interests. Though there is often feelings of regret, only around 5-6% will remarry each other.
Ultimately, I know you know it is their decision, and it may be the best you can hope for is an amicable separation with no hard feelings. Hard to do if the desire for divorce isn't mutual but helping them work toward that if it is inevitable may be in everyone's interest.
Quoted from EricHadley:Just live as a threesome
I have seen that before.... sometimes it works, but it usually does not (at least not happily).