Most Helpful Girls
Just pull them aside in a compassionate way and explain that they may have a slight odour today. If they already know they might appreciate your honesty instead of avoiding the elephant in the room. But be sure to maintain their dignity by not pointing it out in front of other people, it should be in a private place and without any judgement.
You give them a basket with shampoo, conditioner, soap, perfume and all that stuff and say I wanna know what you think get started
Most Helpful Guys
If it is a girl I have never particularly pointed out explicitly as they start crying and angry.
For guys you just tell them straight and or with hints depending on friendship level
'bruh go get a shower'