hi cagecleaner, i would do it, i used to bring my birds to my aunts for playdates with her and my cousin's birds but they don't speak to me anymore(old family issues)but if they needed me to i would watch their's.
no plucking is not contagious, it's caused by different things like taking a parrot from parents to be hand fed, bordum,stress,or a vitamin deficiency.
If the birds a plucker I don't think your other birds will pick up on it. Has your friend had the bird to the vet to rule out medical conditions? Personally if you have to have the bird at your home then keep it in another room away from yours. The better option would be to look after the bird in its own home but thats just what I would do. Its up to you really on how big or small you feel the risk is.