Lassi is a blended yogurt drink, usually full-fat yogurt thinned out with milk or water and then sweetened. You see it a lot at mamak stalls and Indian restaurants, sitting next to the teh tarik and Milo ais. The mango version is the crowd favourite, and at 260 calories per 300ml glass, it is more like a light dessert than a drink. The yogurt brings some protein (8g), which is actually decent, but the 18g of sugar and the mango puree push the carbs up to 42g. To be upfront, these numbers are estimates based on standard recipes because there is no solid lab data for the specific version your uncle's mamak is blending up. The actual calories could be 20 to 30 calories higher or lower depending on how heavy-handed they are with the sugar.
The biggest variable here is sugar. Some places dump in two tablespoons of sugar per glass without blinking. That alone can add another 60 to 90 calories. The size matters a lot too: a generous restaurant pour at 400ml bumps you to 345 calories without changing anything else. If you want to keep it lighter, ask for the salted lassi (namkeen) instead, which is just yogurt, water, and salt at around 110 calories. Or go for sweet lassi without fruit at 175 calories, which is decent if you want that yogurt flavour without committing to a full mango lassi. The main lever is always the sugar, so asking for less sweet is worth doing lah.