Yes, thanks, it's always helpful to refer to average lengths rather than maximums. I think a lot of internet references get the two muddled.
We're sure this lovely guy is not a hybrid Burm/Ball, but rather a pure Burm.
Just as an update, my friend has decided to try to keep him. She tends to feed all of her snakes 'lean' so he will be slow-growing; there is at least a reasonable hope that he won't get huge; no matter how big he eventually becomes, he isn't going to grow quickly so she will have plenty of time to evaluate how things are going with him; her two sons also adore this snake and are committed to helping out; we have a good mutual friend who is experienced with the giants and who will happily take him if it turns out that she feels she can't safely manage him.
I am really hoping this works out for her. She deals with a lot of snakes in her rescue work and has kept for a long time, but I am not sure I have ever seen her connect so much with an individual snake. And he definitely seems to be strongly habituated to her and trusts her. Fingers crossed.