I think I know what Balaji told Radhabai - the details and foreshadowing were enough to hint at what might have happened. And unfortunately, this unexpected, unpleasant revelation will probably end up being used to draw parallels between Radha's and Kashi's marriages when Mastani, "The Other Woman," does show up. I can visualize it now...and it makes so much sense, if the show makers want to twist this into a Radha/Kashi versus Baji/Mastani thing in the end. It would also justify the rift between Balaji and Radhabai, which was hinted at in the lines for adult Bhiu's auditions. That Balaji/Radhabai don't talk to each other.
I had wondered what could cause a rift in a relationship as strong as Balaji/Radhabai's. What could possibly distance two people so deeply in love with each other? I personally don't believe that anything could keep them apart physically and emotionally, from the way their relationship has been shown over the past 100+ episodes...
But the show is taking a leap, so anything goes now - and therefore, this dramatic secret revelation had to ruin the one relationship that could weather any storm. Perhaps this is a useful storytelling tactic. But I'm not liking the way this is shaping up, to be honest. I'm not liking it at all. The story and the characters I love so much will now make way for a new story and a new set of characters, essentially. I wish them all the best...but what a price we had to pay...