I'll have to go and look at the related chapters, although I can't recall any mention of Saruman making a direct offer to Théoden. I agree with the others though who said, Théoden's desire to "speak" to the enemy isn't a direct indication that he never did so either.
It could be Théoden didn't think of Saruman as "the enemy" before when Saruman made his offer. Saruman is talking about some vague time "long ago" and Théoden would have only seen Saruman as an enemy after Gandalf came and revealed Saruman's evil. I guess what I'm getting at is, Saruman talks about a time "long ago" but Saruman was only revealed as "the enemy" only a short time ago, when Gandalf & co. came to Edoras (even though I'm sure Théoden figured out Saruman had been playing him false for a long time).
We know some details about the fall and demise of Saruman, but not much when he switched from being friendly and fair, to actively trying to take control of Rohan and over the region. If I remember correctly, there is a mention that during Thengel's reign Saruman started giving trouble to Rohan:
Quote:
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It was soon after Thengel's return that Saruman declared himself Lord of Isengard began to give trouble ot Rohan, encroaching on its borders and supporting its enemies. ~Appendix A: The Kings of the Mark
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I don't think at this point Saruman's intentions and treachery would have been known, it's just a factual statement that he begins making trouble for Rohan during Thengel's reign. Unfinished Tales makes clear Eomer and Theodred did not trust Saruman, which is why he arguably spends way too much effort and resources on sundering them from Théoden (and trying to kill them) and putting Grima forward as Théoden's trusted counselor.
I do think it more likely that Grima would have been the one (under orders from Saruman) to deliver Saruman's offer to Théoden, than Saruman making the offer to Théoden in person. It can be assumed Grima would travel between Isengard and Edoras (in
UT: Hunt for the Ring there is a mention of the Witch-King intercepting Grima on one occasion when he was going between the two places. Although, I can't speak about the canonicity of this part). From Saruman's perspective, it would have been an offer proposed by him (he wasn't one to credit Grima for anything, except for killing and eating Lotho). And in Théoden's perspective, he blamed Saruman more for the wrongs done to him than Grima. Théoden looked at Grima more mercifully and trying to absolve Grima of most of the blame. Théoden placed the blame solely on "the enemy," Saruman.