If you had two female fancy mice and one died is it ok to put another female in so the remaining one doesn't get lonely or will they fight?

As long as the remaining mouse isn't sick (possibly from what they one that died, died from) then you should get a new mouse to keep your remaining one company! Mice are very social animals and need company of their own species or they will get lonely which can affect their health and happiness. When you introduce two mice together, there can be some fighting as they work out the 'hierarchy' (who's the dominant one). This is normal and shouldn't stop you from getting a new mouse to keep your current one company!

But it is important to change the litter and wash all of the supplies/cage. Also, you want to introduce them in a separate environment. Doing those things will help to eliminate the potential fighting the above user mentioned. Because you want to be sure that there isn't going to be a dominance issue. Mice, too, have their own unique personalities. it is a good idea because i had two mice and one died and then the other one died of lonelyness. mice CAN die of lonliness so it is a good idea to get anotherone as long as they are fairly similar in age