These subtle electrical vibrations are commonly felt especially during meditation and pranayama. Learning the Khechari Mudra is actually easy. The more important element here is the spiritual preparation in which the guidance of the teacher will be of huge benefit.
When the Kundalini is awakened properly, it could be directed to enter the Sushumna nadi where the chakras of the subtle body exist. She irrigates her pathway as she makes her way up, loosening the knots of ignorance known as the granthis, which in turn illuminates the soul.
The tongue holds the key to experiencing God. Energetically, the tongue has a direct connection to both the origins and endpoints of the energetic channels, from the hips to the inner brain, from the Muladhara chakra to the Sahasrara chakra.
Promoting compassion, goodwill, kindness and service bring out our highest spiritual potential, Sattva (Sattwa). This positive, calm and content predisposition, is inherent to all and could be practised even without the physicality of yoga.
He knows and acknowledges his limitations so he could willingly and easily get detached from this euphoric experience. Beyond this is not wise. He needs to go back for him to share this beautiful spiritual journey.