One snowflake doesn't make a blizzard. Biginge. If there were one person making a wrong decision at one BoA branch, that in itself implies nothing about BoA's corporate policies. That's the whole point. If you're willing to blame the whole company for what you assume to be true, I think that's unfair. As BoA also states, they are not free to disclose even the identity of the "customer" in question, nevermind whatever particulars there might be about the circumstances and actions surrounding the anti 2nd amendment claim, which I see is very likely an unwarranted exaggeration. Cowarts's post proves that. Whereas the customer—any customer— has no such restriction. Somebody would have to sue somebody else to bring all the facts to light here.
Therefore, nothing proven. Simple as that. However, if I were in your position, I'd probably back my friend just out of loyalty. After all, BoA is only a bank. The downside here is that encouraging widespread disfavor toward BoA may well harm an organization that demonstrably supports 2nd Amendment Rights, including your own.
Snopes even has an entry, "Undetermined".