Any tips on how to get them both fed?
Personally, if I was in your situation, this is what I would do.
I would start by moving the two food bowls to separate locations in your house.
Then, at the regular feeding intervals for your dog, give both dogs their food.
Your dog should eat all of his food right away. The other dog will take their time, but since you have three days, there is not enough time to fully train the other dog to eat routinely (and besides, it's not even your dog), so you should give him 30-90 extra minutes with the bowl out. It's okay if he doesn't finish, but I would try to give him as much time as possible.
I would do this in an off-limit room for your other dog (e.g. the laundry room) so your other dog doesn't follow. Or if this doesn't work, you can put a temporary dog fence around the other's dog food bowl and not let your dog in.
If this is working, I would keep the other dog's bowl out for 90+ minutes to graze on, what he will have been used to at home.
My only thought is to separate them and try to encourage our friend's dog to eat. If she doesn't, then take the bowl away and try again later.
Again, I don't think it is smart to try and retrain a habit in only three days and with a dog you don't own.
argostraining.com supports this:
Dogs form habits much like humans do. A habit is something your dog does without thinking about it. It can be a behavior that your dog has always done, or it can be a behavior that your dog has been doing longer than 30 days.
We’ve been told (and I’ve repeated in my books ) that it takes about 21 to 28 days to learn a new habit.
In conclusion, make sure to separate the dogs' food (so your dog doesn't just gobble up the other ones), and to give the other dog plenty of time to eat (without the other dog contesting for the food, hopefully in an off-limits zone).