My first and only gliding experience was ruined by the instructor having overpowering garlic breath, it was indescribably awful being encased in a tiny capsule with this guy. It put me off gliding for ever.
Worst smelly student was one whose feet were encased in trainers - without socks - on a typical Perth summer day (a balmy 38 degress). My eyes were literally watering and I realised after I should have just cancelled the flight as the smell was unbearable and I was actually holding my breath rather than breathe in this guys stench.
Next worst is our friends from cooler climes who don't realise the need for extra vigilance in personal hygiene in a hot climate...
Telling someone they smell is one of the hardest things to do, would be interesting to hear how fellow instructors tactfully suggest a student have a shower/brush their teeth/change their clothes before a lesson?