Is Dino responsible for the Lara-Kelly split?

Yes
This is what Hindi films are made of - falling for the best friend's girl. Guess that's what happened with Dino. Probably Kelly in his wildest dreams never imagined what was cooking. No wonder Lara, who is usually so vocal about her personal life, preferred to keep mum about the break-up. As it is, the relationship was not going through the best of times and Dino's entry into the scene sealed its fate. Guess Dino believes everything's fair in love and war. Maybe Kelly wouldn't agree with that at the moment.

No
It is completely unfair to blame Dino for the Lara-Kelly breakup. Things were not hunky dory between them for a very long time and Dino went to the extent of opening the doors of his home when Kelly needed a place to stay after the breakup. Remember how, only after Kabir Bedi left the late Parveen Babi, that his friend Mahesh Bhatt fell for her? The same could have happened here. Lara is also a mature decision maker so no point blaming Dino for the whole mess. He is definitely not the prowling kind.