This is a halacha question, not an opinion question.
When is it “loshon hora” to tell?
When is it “lo sa’amod al dam reyacha” not to tell?
Is there a “right time” and “wrong time” (before they first meet, or after x number of dates) to tell?
Are there other factors to include in the “tell” decision (i.e. he promises to quit, she has asthma, etc.)?
“davidz”s approach – ask a shaila – is the correct one.