Indigo Airlines — Denial of boarding the flight and hence missed

Hi. This is on behalf of my parents Mrs Sunita Agarwal and Mr Brijmohan Agarwal who missed their flight 6E 109 on 13th of March'17.
They had checked in at 5am (Flight bound at 6:20). The gate # was 11B, and they were sitting beside the gate adjacent to 11B (perhaps 10). They are not educated so they didn't know about little details regarding boarding gate.
Their was no announcement made, no phone call made and no msg for them to board the flight on delay in their presence. But still they showed up by 6:05 when no announcement or call for them was made. They were refused entry and their luggage was unloaded !!!
How can an airline leave when 2 passengers have checked in and not showed up? Most importantly, they didn't receive any message or call or no service personnel came looking for them!. They were cashless and homeless that time and they had to come to bangalore and so they had to buy another ticket to come here. Is this fair? They are in their 50s and this kind of experience was delivered to them by Indigo on their first ever flight journey!
What kind of service is this? Once a passenger has checked in how can the flight unload their luggage and leave them and even when they were at the gate by 6? is 5-10 mins of that imporatnce to you all? This is not the kind of service we expect out of Indigo!!! Let me tell you there are bigger flights with better services than your airlines.
I want this complaint to be taken seriously and given a response. I want a refund of my ticket fare else we will move to consumer compalint forum.