MS, I have been in the BIL comes to stay and outstays his welcome situation. U can take a desi man away from his family, but u can't take his family away from him. So something to understand, desi man will always want to help his family out.
Second issue, Yes, while its true that its not right for a person to say yes and then change his mind, sometimes these things do happen, and we have to deal with the consequences.
He will either change his mind, or NOT change his mind. Either way, u have to deal with it. U can turn around and tell your friend that sorry he's changed his mind, or rant and rave and fight over it, trying to get him to change his mind.
If I were in the same situation as u rite now, and husband didnt change his mind, i would tell the friend the truth and I would expect her to respect that. The truth is that my relationship with hubby is much more important than my friends or what they think of me.
Having said that, have u actually sat down with your husband and explained in a CALM manner how u feel? Tell him that u need 10 minutes of his time, and u only want him to UNDERSTAND your perspective, and u are doing this to change his mind, u just want him to understand u, and after what u have to say, if he wants to keep to not letting her stay, then u will respect that, but u hope that he will think about what you want temporarily.
I know u feel frustrated at the mo, but uno maybe overnight he thought of negatives and he may well have reason to believe its not such a good idea.