I'm not a man, but I'd like to put my two cents in the pot. First of all, you're correct in that you're entitled to half of his retirement, social security, and probably anything else he's acquired during your more than quarter of a century marriage.
Second, unless you've been spurning your husband's need for companionship and/or sexual advances on a regular basis, him going outside of the marriage for such things is his fault, not yours. If your marriage is stale, you're not the only one to blame. As a matter of fact, him going outside the marriage is probably a huge contributor to the staleness, because he's not focusing on you or his relationship with you.
Third, if your husband is being conned by a gold digger, you owe it to yourself and to him to wise him up. Why should either of you suffer any financial loss over a bimbo? I have seen this happen so many times: some sweet, young thing who leads with her crotch cons an old geezer into leaving his faithful, loyal wife all because said sweet, young thing is too stupid, too lazy, or both to make it on her own. The husband files for divorce to be with Little Miss Hot Pants only to come running back to his wife after he realizes:
a. He can't keep up with her in any way.
b. She's using him for financial gain.
c. His assets will be cut in half.
What I find funny is that in every case I've seen this happen, the wife decides to follow through with the divorce. It 'aint just widows who are merry! Talk to your husband now, before he goes off the deep end. You don't have to be mean, but letting him know that you're on to them is your right and oblgation. Aside from that, if their relationship is physical, you need to get checked for STDs. He may believe that he's the only one for whom Miss Mary Jane Rotten Crotch spreads her legs, but no one else is the same kind of fool. Sigh, I guess there's no fool like an old fool.