They can unanimously say that they have overhead Tannu saying the baby is not Abhi's but she is still trying or marry him for his fame and name.
Abhi will first be in shock, disbelief and anger.
And after the moment passes, he would have either of the two reactions.
One he will say he never remembered or intended to make that baby and now it makes sense to him why Tannu is behaving odd. Then matter closed. He will know how to show Tannu out.
Second reaction can be, Abhi will say he can't believe it. The team can say he can test it out with DNA but he has to make sure Tannu doesn't realize. If Abhi still argues he can't believe Tannu would do it, they can say Pragya is ready to give divorce, give his property back if his belief wins. Or Daadi can threaten to go to Ludhiana too! Of course then Abhi will do a test on his own.
If Abhi still says he will never ever doubt Tannu, the team can leave him to marry her. But this is impossible since he does have his issues with Tannu and he doesn't want to marry her.
At this stage, it's really ridiculous that the team does not even attempt to tell him the truth but risks lives for no reason.