Why do you think the "Fast" is an issue here?
It's not like the girl is fasting for some religious occassion. It's just being hungry. The morning food you eat is called BREAK-FAST because after a good night's sleep you're breaking the fast.
Here the girl is hungry & Pavan comes to know the she's a non-vegetarian. So the Chicken song happens as he goes in search of chicken for the girl to eat. And a pakka Hindu vegetarian like Pavan searching for chicken justifies the use of the word "Dharam-Brasht".
If Madhavan can eat chicken in Rehna Rehne Terre Dil Mein / Minnale being a Hindu vegetarian & later vomiting it out having nobody raise an issue then there's no problem with BB's Chicken song too.
It's not like the Telugu film Denikaina Ready where a muslim guy disguises himself as a Hindu purohit & just for comedy sake, a couple of muslims make all the Hindu brahmin purohits eat chicken leg piece. Brahmins made sure they protested heavily for this film & especially for one of the supporting actress playing a Brahmin wife vulgarly.