Maybe you've already done so, but do make sure that you talk to your GF about it. Chances are that it's not a problem for her, but she may have some opinion about "a stranger" coming on the property to do work while you're away. Not likely to be a problem that would stop you from doing it, but she might have some opinions about who it is. And, how does she feel about taking care of the dog while you're away? Feeding, walking, letting it out in the yard. Any chance that the dog will have trouble waiting until she gets home to go out, or get destructive in the home due to boredom or anxiety? Figure out how it will be handled if the dog gets ill or injured, how will she get it to the vet, will she know what kinds of things to look for? Etc.
Anyway, I agree that as it's your house, not "yours" plural, and the fact that you usually take care of these things, and with her schedule, that it makes sense to hire it out. Or, if you really think that your friend would be willing, then ask him, though make it clear that you would want to compensate him for his time, and regret that taking care of his dog wouldn't be the way you could do it. He might decline to be compensated, but there are ways to do it anyway - gifting him something, or helping him do something at his place he couldn't do alone (build a deck, etc).
Also, you didn't really ask this, but if you want advice about how to save the maximum amount of your per diem, there are definitely people on the forums who know how to do that! Different strategies abound, depending on the facilities where you're staying. I'd post a different thread for that, if I was you, with a catchier title. "Maxing out my per diem!" or something.