Should you let friends holiday in your property?

Having bought our lovely apartment two years ago near Guardemar del Segura and shown it off to friends and raved about the unspoilt (mainly) area, we were asked by a friend if he could go there with a friend for a week as he didn't have a lot of money and hadn't had a holiday in 3 years. The heart strings were being tugged already, and I thought I could try to keep any costs down that I could, after all he hadn't had a holiday for 3 years. Let's call him Justin for the sake of a name. The rate I came up with would cover any utility bills, and also after some research on holiday letting sites be somewhat cheaper, by at least £100 pounds, on condition he gave the place a good going over and left ship shape. So far, so good, Justin over the moon with the rate, even commenting that an apartment in our very own community w a lot more expensive and our furniture etc was so much nicer compared to the standard in the others, he had done his homework! Had a nice email after he arrived, all was well, he was delighted with everything and he and his partner just adored the area. On the day he should have arrived home, I called him to see if all was ok, funny I got a euro tone on his phone, oh dear I thought, he's left or lost his phone in Spain ! next day the same but no answer to my call or text. This was now day nine of a seven day holiday. The following day I do get a call, with some fantastic story how the bus to the airport broke down, he missed his flight and returned to the apartment and couldn't get a flight back for two days to London from Alicante, even I was surprised, with all the flights available. Didn't think to let me know he was staying two extra nights, but I sympathised a little with his plight. No offer to pay for the extra, but then he had to pay out for new flights so I did feel sorry for him. Now it gets interesting, I happen to be very friendly with a work colleague of his, as I used to work at the same place until recently and Justin was a friend through work. I mentioned how Justin had had a terrible time getting home, this was met with surprise as she said he had booked off to return after a break of 10 days, and had told everyone his plan was to return on the so called rebooked flight schedule! So it transpires he paid me for 7 nights but had a 9 night ! A call to a neighbour in Spain confirmed he did not leave on agreed day. On top of that I have a no smoking rule and this neighbour popped in to check, all ok and the place reeked! Can't wait to get utility bill, probably what I charged him won't cover him no doubt leaving air con on day and night. Lesson learnt, I am yet to confront him with my knowledge, when I have calmed down! Now another friend has asked, and I cannot decide wether it's worth the aggravation for about £80 pounds profit, which I put into our maintenance fund. Any tips for a naive owner much appreciated!