As far as I know, there's no real knowledge yet about blood groups in chameleons. But that doesn't matter for one single (and especially the first in a patient) transfusion. As in every other species, your vet will do a crossmatching test before transfusion to assure donor's and receptor's blood don't agglutinate (major crossmatching = donor's erythrocytes + receptor's blood plasma, minor crossmatching = receptor's erythrocytes + donor's blood plasma). You can gather up to 10% of the blood volume of a reptile, which means about 1% of its body weight. E.g. a Chamaeleo calyptratus male weighing 180 g could spend about 1,5 ml of his blood to another chameleon without problems. Just some short thoughts, perhaps it can help assure you it's worth giving your female this chance. If your reptile vet hasn't done a transfusion in chameleons before, he could phone collegues like Dr. Mader for further advice.
Some publications exist about blood transfusion in reptiles (alligators, tortoises), but unfortunately it's still a few experienced topic. There was a nice article in CIN 2001 about a transfusion done in a Calumma parsonii easily explained for keepers, if you're interested I could send a copy to you.