Mark field mandatory only if ‘yes’ option selected in another field

In the middle of a very complex form used in a meeting booking system, I've been asked to create the following

Field 1: Text Input

Field 2: Yes/No radio

The project BA is requesting that first field be mandatory, only if the second field is set to 'No', otherwise it should be optional.

Filling the first field will be used to help populate the meeting title. The user then gets the option to also edit that at the end of the process.

I think this is a very poor idea, but they are insisting on that order and that the first field is only mandatory base on the answer to the second question.

Given that I guess I'm asking two questions, what would be the best way to deal with a mandatory field indicator and what are the best arguments against doing this?