Leaving aside the usual 'Phoowargggh, go for it my son ...!!!' level of Neanderthal comments which this type of thread always attracts -
this is not appropriate behaviour by the T.A.
There is a time and a place for a mature woman to gently tease an attractive teenager, but in the classroom, when he is a pupil, and she is a TA is neither of them.
Adult staff bear a huge responsibility in daily dealings with maturing and impressionable teenagers, so they have to be seriously careful that their behaviour is above reproach all day every day - it goes with the territory.
If this woman is too immature to realise her responsibilities, she should be advised soonest - in order that another similar doubtless innocent remark does not lead her down the path to career-stalling dismissal.