Awwwww..... poor little guy. Did she happen to give you a water test kit?? It would be good to know how much ammonia is in that tank since you don't know what her water changing schedules were (I doubt they were frequent).
I would do dialy water changes of about 15% until you think you've replaced most of that possibly toxic water. Changing it all at once might be too much of a shock for the fish since he's acclimated to the water he was living in. Don't forget to condition the new water with something like Seachem Prime to remove chlorine before adding it to the tank.
Does he have any hiding spots like a little cave or leafy plants? That will reduce his stress.
And yes, he's a male. A veiltail. Keep an eye on the fins and look for signs of rot. I'd say to add some AQ salt to the tank but I think the other two fish might be sensitive to it.
Good luck with your new friend.
Animal testing is a terrible idea; they get all nervous and give the wrong answers.