and then steelmanned it by carefully investigating the conditions of when TB is likely to be transmitted

I don't see how this constitutes steelmanning.

If his mother's allegation is "You'll be at risk of tuberculosis," then a refutation of her argument as she presented it demands that he assess the risk of tuberculosis he'd be subjecting himself to, and demonstrate that it is in fact low.

A steelmanning of her argument would, as Luke indicated, entail addressing not only her concerns about the risk of the disease that she mentioned, but also address other hypothetical risks which would appear plausible.