Why? the Argentines were Rebelling BECAUSE, they feel, the Spanish left them completely abandoned to foreign attacks, don´t let them commerce and put them against British designs, so if the British take Buenos Aires, supported by the Spanish or without spanish protest of the fact, you just put two legs on the royalist side stool, if anything this will see a major Revolutionary fervor and independentist sentiment, The Royalist argued that the best protection the country have against their enemies , indians and pirates, is maintain themselves inside the Empire, f the empire can´t defend them. Why be part of the empire?
on the other hand remember nothing unify more than a external attack to one country, we could see royalist and independentist forces uniting to repulse the British from Buenos Aires, And the Argentine become even more insufrible as now their independence war will be Against the Spanish and the English.