Hí would be acceptable if you knew that your lawyer were a woman, but didn’t yet know her name. If you didn’t know anything about your lawyer, then an epicene hé would be used. Given the lack of context, an argument could be made for also accepting hí in this sentence.
That's not lenition. That 'h' is only there because 'cé' and 'ní' end with a vowel and 'iad', 'í', and 'é' start with vowel. Otherwise, you'd end up with hiatus between the vowels when speaking, which Irish tries to avoid.