I am with others, and would like to add, best not to even bring up a 'problem' before it is indeed a reality. Then the whole 'refund' scene need not have had to come up. I have removed from my hosting vocabulary the word 'refund' btw; too easily said by hosts and too easily abused by guests.
To the matter at hand: They are arriving Friday, you have realistically 4 days to get a smaller, simple 'backup' one up to your place and bring the one to be fixed back down. Upon arrival, if the backup is small and watching their reaction, you could always say (or mention if not there or they make mention)- 'That is the backup, the regular one just went out a few days ago, thank God we had a backup' - they will then be grateful you had a backup.
Btw, I spend all my kid and college summers in Phoenicia, always think of it with the fondest memories. You have a lovely place. For some inexplicable reason $195 a night (not $150) for 4 people struck a bell with me, considering it is a summer place (limited season). The instant mention of a refund (by the guest) also struck me odd, have many of your guests given you the impression they are bargain hunters? If so, your place may be priced too low.