After Happosai managed to escape from his prison and made his way to the Tendo dojo. Fearing that he was out for revenge, Soun and Genma immediately tried to kill him, but after being instantly and effortlessly defeated they groveled before him. Genma came up with a lie about how his dishonorable son had left and abandoned his responsibilities to the Art the old perverted grandmaster of Anything Goes left promising he'd find Ranma and bring him back to face his duties.
Happosai was 'training' when he felt a huge concentration of energy and was surprised that it was Ranma. He would've continued to be in awe at how Ranma was absorbing the energy of nature when he caught sight of Aeris and Hinako. Any doubts he had about making Ranma his heir were gone from his mind with that sight. If the boy attracted fine specimens of feminity like those two he was more than worthy enough to be Happosai's bait... err... pupil.
After five minutes of jumped around Aeris performing feats of dexterity that should be impossible for someone that old. Happosai took pity on the girl and began studying Ranma. While Ranma was brimming with chi he had the physique more of a body builder than a martial artist. When he brought this to Ranma’s attention he was shock when this was met with attention. Not having expected a son of Genma's to take so well to an insult of such magnitude, Happosai was left open for the recovered Aeris to get the drop on him. When he woke up after the rather savage beating he berated himself for being distracted. That and reminded himself to ask Aeris just where did she learn her techniques, which were quite advanced for someone so young.