You should tell him that this behavior is not allowed, but in dog language. When humans get angry, they often start hitting or slapping dogs, but this is less effective than other gestures and in the worst case can lead to aggression. So please don't hit your dog or cause him pain in any other way.
Yelling or screaming at the dog is also not effective. It doesn't matter if you yell "stop" or "no". The dog won't understand you and won't stop.
Redirect his behavior whenever possible. The moment he comes to your mom, offer him either a treat or a toy. If he's interested, guide him away from your mom with the object and only let him have it if he's at his dog bed or a comfortable distance away. This is the best possible solution because it involves positive reinforcement, but I'm well aware that it can also involve the most effort.
A rattling bottle is probably the next best solution, but there are disadvantages as well. You take a small empty plastic bottle or metal can and fill it with a few screws and nuts or stones. When you shake the bottle, it makes a loud rattling noise. Your mom should hold the bottle and rattle every time the dog starts licking her or humping her. Don't allow him to lick her a few times before you rattle, do it at the very first lick so he can learn that licking is not allowed.
The disadvantages are that the dog might become afraid of her and keep his distance. Maybe your mom likes it, but maybe she wants to cuddle hith him later. He probably won't come to her for cuddles or to play.
For small transgressions it might be enough to turn his head away. Gently lay your hand over his snout (the part of the head in front of his eyes) and simply turn his head away. He will probably try again a few times, so repeat the gesture but hold his head in the turned away position for a second longer. If he still doesn't stop after 3 - 4 times, use the rattle bottle.