Poorly hen

Hi I re homed some ex batt hens last April. One died after a month. The other a few months after unexpectly and my third has had an infection which the vet gave me some anti biotics to inject in her neck for four days. She seemed to of perked up. This was two weeks ago. Today she just hunched over and hid her face in corner of run. I put her under heatlight in shed with food and water which she ate some crumb and watermelon. She came out for a short period but I can tell she is very poorly. I rang my vet again but not there till later. Would it be kinder to have her put to sleep or are they happy to live like this until they pass away. I know if she goes to vet will be put to sleep but if she is happy to live out this way fine. I will then be left with one will she be ok alone as ideally don't want to rehome more. I love having them but find them quite expensive with vet fees, watermelon, banana, sunflower seeds, cleaning materials weekly etc. I know food treats optional but they deserve a treat what they've been through. so should I have her put to sleep or keep her. Will my other be ok alone if I put mirrors toys etc in. Thanks for time reading and replies. Jo