From the above link the following has been taken
Calling a Dalit by his caste with a view to insulting him or her is an offence under Scheduled Castes and Scheduled Tribes (Prevention of Atrocities Act), the Supreme Court has ruled, upholding the conviction of two persons in Tamil Nadu.
In some parts of the state, there is a practice of serving food and beverages in separate tumblers for members of the Dalit and other castes. The accused Arumugam Servai and Ajit Kumar, who belonged to the 'servai" backward caste community, were convicted by the sessions judge Madurai for calling the Dalits Panneerselvam and Mahamani by their caste "pallan" and assaulting them.
"Even calling a person a 'pallan', if used with the intent to insult a member of the Scheduled Caste, is, in our opinion, an offence under Section 3(1)(x) of the Scheduled Castes and Scheduled Tribes (Prevention of Atrocities Act), 1989," Justice Katju writing the judgement said
From the above lines it can be seen that calling a dalit as pallan with a view to insult him or to assault him or in a derogatory sense, it can be termed as insult and hence the punishment as found in the said article.
However you have stated that one of your friends while talking with the father of the girl he was engaged during a heated exchange in phone he told him "There is no use in talking to this Pallan".
This was not planned insult or derogatory hence it cannot be termed as an offence, moreover your friend talked to him over phone alone which has not been witnessed by anyone, hence it will be difficult to prove the offence even if booked under pressure.