If I was the one being stolen from, I would be glad my friend did it. They and anouther person can have a joyous vacation for such a small cost to me. A lot depends upon the relationship you have with your friend. Now a stranger, that would be a different story.
At the end of the day, stealing is stealing whether £1 or £100; no form of stealing is acceptable.
Your argument might be: he doesn't need the £10, he can afford to live without it. Well, I would say that whether he needs it or not is irrelevant, because think of this; when you rob a bank, you are not robbing a lot compared to the amount of money the bank has but you still get jailed for it.
Next to clear up is that it wouldn't be right to steal from them, even if he is attending the holiday. If you want £10 from him, the best option is to just ask him.
(Just to clarify, I am not being 'sexist' by using "he", it is simply the term I am using in this instance)