I'm all for helping friends, but anything less than the formal contract with specific stipulations could easily result in a very sticky situation if those "friends" cancel, say, one week prior, and leave you holding the bag if there's no formal contract. Without a contract, the friends would certainly expect to be "let out of any obligations", and would not expect to pay ANYTHING, "because we're friends".
I've seen similar situations many times, so imho, it's ALWAYS best to spell things out well before,
and thus PRESERVE a friendship rather than end one.