The rule is that whether a subject is singular or plural has nothing to do with subordinate clauses. So when considering, "Suresh, along with his friends, was arrested ...", the "along with his friends" is irrelevant. If you just left that out, you'd have, "Suresh was arrested". Singular subject, singular verb. But I guess you understand that part.
In the second part of the sentence, "... as they were arrested", the subject is not "Suresh". The subject is "they". And "they" is plural, so it requires a plural verb.
You're getting confused because the "they" presumably means "Suresh and his friends". Not "Suresh, along with his friends". There's no rule that says that such a clause must have the same subject as the main clause in the sentence. Suppose instead, for example, the author had written, "Suresh was arrested as he and his friends were caught in a sting operation." "He and his friends" is not the same as "Suresh". Or, "Suresh, along with his friends, was arrested as the police conducted a sting operation." Clearly "Suresh, along with his friends" are not "the police". This is a totally different subject. Or, "Suresh was arrested in Bombay while his brother in London was sleeping." Two totally different people doing totally different things.
Just because in this particular case the subject of the second clause happens to be the same group of people referred to by the subject of the first clause plus a modifier, doesn't mean that GRAMMATICALLY they are the same. Don't confuse what is factually correct with what is grammatically correct. Like if I said, "The people who assassinated Senator Jones are part of a vast conspiracy", and in fact the truth is that Senator Jones was murdered by one man acting alone, that does not mean that the correct grammar of the sentence is, "The people who assassinated Senator Jones IS part ...", because there's really only one person. :-)