I agree that for something like this, it's unreasonable fo your husband to refuse to go with you simply because he has to sit at a different table for the meal. After all, even if you were simply attending as a guest, not a BM, you would both be expected to mingle, and talk to other people.
My sister got married earlier this summer. My (other) sister's boyfriend came to the wedding - they've been together a relatively short tiem, an live some distance away, so he had not met anone except members of the briadal party - he came, sat at a different table for the eal and speeches and had a good time. As did my brother's girlfriend (he was in the wedding party).
It's fairly common att weddigns to end up with people you don't know, or only know slightly - you just get on with it.
If your husbnd is particualrly anxious, is there anyone he does know who is likely to be going? Do you know / socialise with any of the other bridesmaids, fr instance - could you organise a meal out or something and invite them and their partners so he can meet a few of the other wedding guests and feel more comfortable going in.
I think that if this is a real dealbreaker and your husband can't or won';t compromise then consier going alone to support your friend, rather than declining to act as BM. It may be less fun than going with your DH would have been, but this is your best freind and although she sounds like sometone who wouldn't make a fuss, I think turning downher invitation to be a bridesmaid is likely to hurt her feelings, especially as the reason is one which, to an outsider, seems fairly pettty. (I am assuming that your DH does not sufer from any erious anxiety disorder or other medical condition whcih would make not sitting at your table a particular issue for him . If he does, then I think that explaining that issue to the Bride would be reasonable. You are then able to explain that you'd love to be a BM but that DH has a medical need for your support, so you will have to decline but hope very much you'll both be welcome as ordinary guests. The Bridge can then, if she wishes, offer a compromise for you to be seated with your DH instead of at the 'top table'.