Headlights problems of the 2009 GMC Envoy

For 5 months the headlights work when they want to, the first couple months they were working more than not, last couple months they were not working more than they did work.

Low beam headlights don't work even after replacing the hdm.

The headlights go out and will not come back on while driving my sister took it to gm while it was still under warrenty and was told that is not covered under warrenty she paid $35,000 for this GMC and can not drive it at night 6months after buying the Envoy if my sister gets killed because of this unsafe Envoy its on GMC if it comes on a GMC and its still under warrenty then it should be covered,if GMC sell defective equipment then it should fix it if it under the GMC name.