Subject to moveable arm rests will certainly apply.

However, given that it sounds like they only want to make you buy 1 extra seat in ET, not 2 extra that would make a row, then it would be worth asking, they might not make you buy any thing extra - given that there are two of you and a middle seat free.

If it was just her on her own, I don't think it would be fair to the person in the same row if she had the middle seat space allocated to her (as it were). But if there are two of you travelling as a party you might be able to agree to that.

Just a thought, no info.

(This is assuming you are not in CE on the Embraers out of LCY - in which case she will certainly need another seat as there is no middle seat.)

EDITED TO ADD: Also check the IB policy, as the operating airline for one of the later flights.