Angels can do fine in singles actually. You can get him a friend, or two, if you want. If you do get more, I suggest odd numbers. Some can pull off even numbers, and after a certain point it doesn't matter if you're odd or even(after five or so). But if you want to get him a pair of friends, and if you have space, go for it. Sometimes just adding one more to a single can lead to trouble though not always, so be prepared.
Just make sure the others died from being difficult like the shop said...angels really aren't that hard, so make sure it wasn't an illness or problem in the tank(be it water, space or tankmates) before you get more, and be sure to QT them first. =)