Should a plus one buy a wedding gift?

No. In your case, if you are inviting a friend, if you are giving a gift you may offer to have your friend sign the card along with you. Otherwise, she has no need to be involved in gift giving.

Technically no one is obligated to purchase a wedding gift. Understand that the gift is a gift from the heart. It is not payment-in-kind of the reception meal.

First thing you never EVER ever ask a bride if you can bring a plus one if she did not already offer you that option. It is not only rude but an imposition because they have to pay by the head at the reception venue and you just cost them money they were not prepared to spend The bride said yes because she is far more socially adept than you and she responded with politeness to your rude and ignorant request. Now that you have cost the couple money and they have accommodated your request for a plus one Don't you think it is just fair for that unexpected guest to bring a gift? It would be double rude for a virtual intruder to also show up empty handed

You were really rude to ask to bring a guest, if you weren't specifically invited with one. But too late now.

Your parents may sign the card on behalf of your whole family, in which case you're all set.

If your parents have said you're on your own for a gift, then give cash in a card or buy something off the registry (whatever amount you can afford) and sign your and your friend's name to the card.

Your friend does not need to bring her own gift.

But, really, next time DO NOT ask if you can bring someone.

No, and honestly you don't need to either. Either way, it wouldn't be up to you whether your friend gives a gift.

It might be nice to give them something like a small photo frame for one of their wedding pictures, along with a card signed by you and your friend. You don't have to get a gift, but it's a nice thing to do.

The plus one doesn't need to bring a gift, but it was totally wrong of you to ask to bring her. They don't know her!
It was a rude imposition on your part, so maybe you should double your gift giving to help make up for it,at least. If its not too late, your best bet would be to inform them that she isn't able to come, after all. But if they've already paid for another guest, it won't matter. Please, never do this again.

If you were RUDE enough to beg for extra invitations for uninvitated guests, the LEAST you and her can do us to buy them a proper gift.

You and your friend have no class or manners at all. Mooch!

No, she does not need to. If you are getting a gift then great. She is not the invitee, just the plus one.

a plus one should wear loose clothing.

Yes

The proper thing would be to have the invited guest buy a joint gift or give a joint card w/ a check from the two of you for double the amount you would spend if you were a single guest. You never want to appear to be a freeloader or cheapskate. If you can t afford to do that, then you shouldn t bring a guest with you.

You don't have a plus one. The bride did not graciously say yes, she caved in to your incredibly rude presumption. Apologise to her.

You'd generally buy them a joint gift from you both.

No an plus one is not an invited guest by the couple.
Only invited guests should bring an gift if they choice to.

You should get them the gift, your friend does not get one. Think of it this way are your other relatives that are say married giving two gifts or just one.