well... be honest but use the honesty to your aid...
pretend to be very interested in the lecturer's topic and then try to do something impressive like answering a extremely hard question to get his attention... basically give her dad a GOOD IMPRESSION....
so if you see her again at the next supervision, praise her socially to the lecturer (something like you've got a great daughter sir or somthing) but don't go to the extreme of asking about her yet....
Then see if you can talk to her... and try to be friends... and if it works... he should talk about you to her (many lecturers talk about their day to their children)... so use his good impression to give her the good impression...
THEN do whatever you please the next time you see her... such as buying her a drink / helping to carry her bag / ask for her number... BUT BE HONEST...