My mom has had Chihuahuas for the last 20 years. She has had no choice but to leave her dogs in other people's care when she can't take them with. Your girl may be small but from most of the chihuahuas I have met are more resilient than they look. She might get a little homesick if she is used to having you around all the time but she will NOT forget you. Dogs will remember those special to them. I used to go off to college and be gone for 3-4 months a stretch and be able to come home and know that the dogs remembered me. She might hold a small grudge but get over it quickly. I think that you are doing everything that needs to be done to make her vacation at your brother's friend's place enjoyable. Only thing I would add that you need for this friend is the number to her regular vet. If something should happen (which I don't see happening) he can contact her vet and get her the same care you want her to have from her vet.
I hope you enjoy your trip!
~Pink is Not Just a Color! It is an Attitude!~
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