In this case, I tend to agree with Red. From what I can see of the situation, they are seperated (both physically and emotionally) and she is not showing an interest in working on the marriage.
As long as he approaches this by being up front and honest with this other person, I don't see any harm in having some casual companionship (and I don't necessarily mean sex). Back at the end of last year when things were at the lowest of lows, I met a couple of women that I was interested in. Had a couple of dates and was straight with them about my situation. My W found out about this at some point (I think it was before we started piecing), but my point is that this didn't end up being a deal breaker. (Disclaimer here... we are seperated now but there were three good months of piecing, so I really think these dates were inconsequential.)
It can have positive results both from the perspective that the WAS will see others having interest in the LBS and also the LBS will get a big PMA boost.
I understand that every situation is different and there is the possibility that it can backfire, so this is a very personal decision.
