All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Whoever is consulted about marriage should give the right advice as the “counselor is trusted” as the Prophet said in the Hadeeth reported by Ahmad, At-Tirmithi and Ibn Maajah.
So, as long as you are a doctor and you were asked to give advice, you should inform the questioner, who wants to marry this girl, about any illness you know she has. But you should make sure that the questioner intends truly to marry this girl and that he will not tell anyone else about her health problems. However, one should know that asthma is not among the diseases that needs to be revealed before the marriage contract. Also, asthma does not cause the revocation of this contract. But the proposing man has the right to know about this illness so he can act accordingly.
For more benefit on disease that gives the spouse the choice to nullify the marriage contract (in case they were not informed about it), please refer to Fataawa 365911, 384409, 128182, 134855, 90593, 84591, 326140, 263554, and 195891.
Allah knows best.